avenger − Mail Avenger


Mail Avenger is a highly-configurable MTA-independent SMTP (Simple Mail Transport Protocol) server designed to let you filter and fight SPAM before accepting incoming mail from a client machine. avenger is the script run on behalf of each user to decide whether to accept incoming mail.

When a client attempts to send mail to a user on the system, the avenger SMTP daemon, asmtpd, runs avenger to process the file .avenger/rcpt in the user’s home directory. That file, a shell script with access to special functions, determines how the SMTP server should proceed. The possible outcomes are:

Provisionally accept the mail, falling back to system-default rules

Accept the mail immediately with no further checks

Reject the mail immediately

Defer the mail, telling the client to re-send it later

Redirect the processing to another local name. The name can be another email address belonging to the current user, or an email address belonging to the special AvengerUser user. In the later case, avenger will be re-run with a different user ID , and hence can, for example, employ utilities that maintain state across multiple users (assuming they all redirect processing the same way).

Run a "bodytest" rule. With this outcome, the the SMTP transaction continues on to receive the entire contents of the mail message, after which a program is run on the contents of the mail message. That program can decide, based on the contents, whether to accept, reject, defer, or silently discard the message.

Mail Avenger should typically be configured to have a Separator character, allowing each user to maintain multiple email addresses. With sendmail, Separator is typically "+", with qmail it is typically "−". If the separator is "+", then any email sent to user+ext@your−host will be processed by files in user’s .avenger directory.

Avenger first checks for a file named rcpt+ext in a user’s .avenger directory, then for rcpt+default. If ext itself contains the separator character, for example user+ext1+ext2@your−host, avenger will check first for rcpt+ext1+ext2, then for rcpt+ext1+default, then for rcpt+default. The same algorithm is extended for arbitrarily many separator characters. (If separator is "−", simply replace "+" with "−" throughout the above description, including in the names of files such as rcpt-default.)

If mail is rejected by the recipient checks but the sender address of a message is local and UserMail is 1 in asmtpd.conf (which is not the default), then before rejecting mail, avenger will be run on behalf of the sending user. In this case, the address will be parsed as above, but avenger will look for rules in files beginning mail instead of rcpt. This mechanism can be used by local users who want to relay mail through the server from an untrusted IP address.

Using the mail configuration files, each user can, for instance, configure a mail+... file to accept mail from an IP address he or she trusts, even if that address is not trusted by all users. (Alternatively, using tools such as macutil, a user might set up relaying of mail in which the envelope sender contains a cryptographic code, checked by the mail+... script.)

Error output of an avenger script rcpt+ext or mail+ext is redirected to a file called log+ext in the same directory, for use in debugging.


Avenger configuration files are simply shell scripts, using the syntax described in sh(1). Each line of the file contains a variable assignment, command, or function to run. Scripts can additionally make use of a number of avenger-specific functions and variables. This section describes avenger functions. The next two sections describe variables.

Certain error conditions result in Mail Avenger rejecting mail by default, unless the message is explicitly accepted through an accept or successful bodytest check. These conditions are indicated by the MAIL_ERROR environment variable described below. If your script either rejects mail or falls through to the default behavior, there is often no reason to run tests on a message that will end up being rejected either way. errcheck exits immediately with the default error if the default would be to reject or defer the mail.

accept [message]

Immediately accepts the message (without falling back to any default rules). If message is supplied, it will be returned to the SMTP client. The default message is "ok".

reject [message]

Reject the mail, with message. (The default message is "command rejected for policy reasons").

defer [message]

Reject the mail with a temporary error code, so that a legitimate mail client will attempt to re-send it later. The default for message is "temporary error in processing".

bodytest command [arg ...]

Accept the current SMTP "RCPT" command. However, once the whole mail message has been received with the SMTP "DATA" command, run command with the message as its standard input. Depending on the exit status of command return to the client’s "DATA" command either success, temporary, or permanent failure. Exit code 0 means accept the mail, 100 means reject, 111 means reject with a temporary error code (i.e., defer the mail). See the description of bodytest in the asmtpd/avenger interface description for more information on bodytest (since this function directly invokes bodytest in asmtpd).

Error output from command will be redirected to the same log file as output from the rcpt+... avenger script invoking the bodytest function. Standard output of command will be included as a diagnostic the bounce message if the exit code defers or rejects the mail.

Note that command and the arguments passed to bodytest will be run by the shell. Thus, it is important not to pass any arguments that might contain shell metacharacters such as ">" and "$".

redirect local

Finish processing, and re-run avenger as if mail were being sent to a different username local (possibly belonging to the special AvengerUser user). See the description of redirect in the asmtpd/avenger interface description for more information on redirect (since this function directly invokes redirect in asmtpd).

greylist [sender-key]

This command defers mail the first time mail is received from a particular sender at a particular IP address. However, after a certain interval, greylist_delay, if the client re-sends the mail, it will be accepted. Furthermore, from that point on, all mail will be immediately accepted from that sender and IP address, unless the sender stops sending mail for a period of greylist_ttl2 or more. If, however, after sending the initial, defered piece of mail, the client does not try again within a period of greylist_ttl1, then any record of the client will be erased, and the next time it tries to send mail it will be defered again.

The parameters can be tuned by setting variables in the script. The default values are:

    greylist_delay=30m  # Time to wait before allowing message
    greylist_ttl1=5h    # How long to remember first−time senders
    greylist_ttl2=36D   # How long to remember ok senders

m means minutes, h hours, and D days. For a complete list of allowed suffixes, see the documentation for dbutil(1) (in particular for the −−expire option).

sender-key, if supplied, is used to identify the sender. The default value is "$CLIENT_IP $RECIPIENT $SENDER". If, for example, you wanted to record only the first 24−bits of IP address and didn’t care about the recipient, you could use the command:

greylist "${CLIENT_IP%.*} $SENDER"


All functions that set a variable by means of an external query to asmtpd are performed asynchronously. setvars actually waits for results and sets the values of those variables. In this way, a number of potentially slow requests (such as DNS lookups) can be initiated concurrently, and their latencies overlapped. However, one must remember to call setvars, or else variables that should contain the results of operations will remain unset.

dns var type domain-name

Performs a DNS lookup of domain-name for records of type type, and assigns the result to variable var when you call setvars. type must be one of a, mx, ptr, or txt (lower-case only).

rbl [−ipf] var domain

Looks up the current mail sender in a real-time blackhole list ( RBL ). domain is the domain name of the RBL (e.g., "bl.spamcop.net"). If the sender is listed, set var to the result of the DNS lookup when you next call setvars. −i looks up the sender’s IP address (the default if no options are specified). −p looks up the sender’s domain name (verified DNS PTR record). −f looks up the envelope sender domain name in the RBL .

spf0 var [spf-mechanism ...]
var [spf-mechanism ...]

Tests the sender against an arbitrary query formulated in the SPF language. This is a powerful way to whitelist or blacklist particular senders. For example, suppose you want to accept any mail from machines in the list maintained by trusted−forwarder.org, accept mail from any machine name ending "yahoo.com" reject any mail from users in the spamcop RBL , and for other users fall back to the default system-wide rules. You might use the following rcpt file:

    spf MYSPF +include:spf.trusted−forwarder.org \
        +ptr:yahoo.com −exists:%{ir}.bl.spamcop.net ?all
    case "$MYSPF" in
            accept "I like you"
            reject "I don't like you"
            # Note, could instead fall through to default here
            defer "Temporary DNS error"

Note that commands spf0 and spf are synonymous, but spf is deprecated, because in a later release of Mail Avenger spf will become synonymous with spf1.

spf1 var [spf-mechanism ...]

Performs the same tests as the spf directive, but returns the result strings None, Neutral, Pass, Fail, SoftFail, TempError, and PermError instead of none, neutral, pass, fail, softfail, error, and unknown.


These variables are set by the avenger script. In addition, asmtpd sets a number of environment variables before running avenger. These are documented in the next section, ENVIRONMENT .

The extension on the file currently being processed. For example, if file rcpt+ext is being processed, will be set to "+ext". Empty when processing just rcpt (or mail). May also contain default when a default rule file for some suffix is being run.


Assuming the separator is "+", when processing a file rcpt+base+default or mail+base+default, PREFIX is set to base, while SUFFIX is set to the portion of the name for which default was substituted. When the file does not end with default, SUFFIX is empty. When the file is just rcpt with no extension, both PREFIX and SUFFIX are empty. When SUFFIX itself contains a "+" character, SUFFIX1 contains to the part of SUFFIX after the first "+" character, SUFFIX2 contains the part after the second "+", and so on for each "+" character in suffix.



If Mail Avenger was compiled with SASL support (which is not the default, unless you supplied the −−enable−sasl argument to "configure"), and if the client successfully authenticates to the server using SASL , then AUTH_USER will be set to the name of the authenticated user.


Set to "rcpt" when testing whether a recipient should receive mail. Set to "mail" (possibly after an "rcpt" check fails) when checking whether to relay mail (possibly on behalf of a local user).


The effective local username for which avenger is being run. Ordinarily, this will be the same as:

However, for special avenger files like unknown and default, it can contain useful information, because unlike the RECIPIENT_LOCAL environment variable, AVUSER reflects substitutions from the Mail Avenger domains and aliases files.


This variable contains the name of the client machine, as typically reported in "Received:" headers. Its value has the form:


user is the user name for the connection reported by the client, if the client supports the RFC 1413 identification protocol, otherwise it is omitted. host is a verified DNS hostname for the IP , if asmtpd could find one. Otherwise, it is simply the numeric IP address.


Set to 1 if the client included a space between the colon in the command "MAIL FROM:" or "RCPT TO:" and the subsequent "<" that begins an email address.


If AllowDNSFail is set to 1 in the asmtpd.conf file and resolving the client’s IP to a hostname returns a temporary error, then this variable will be set to a description of the error.


Set to the argument the client supplied to the SMTP "HELO" or "EHLO" command.


Set to the IP address of the client.


Set to the verified DNS name of the client, if asmtpd can find one.


Set to the number of network hops between the server and the client, if asmtpd can get the client or its firewall to return an ICMP destination unreachable (type 3 packet) in response to a UDP probe. Whether or not this is set will depend on firewall configurations.


Set to as many intermediary network hops as asmtpd can determine between the server and the client. How close to the client asmtpd can probe will depend on firewalls.


Set to 1 if the client wrote data after the SMTP HELO or EHLO command, before receiving its response. A correct SMTP client should not "pipeline" commands until after receiving the result of the HELO command and verifying that the server accepts pipelined commands.


The TCP port number of the client.


Set to 1 if the client sent a "POST" command at some point during the SMTP session. "POST" is not a valid SMTP command; it is an HTTP command. However, one technique for sending spam involves exploiting an open web proxy to "post" an SMTP session to a mail server. The initial HTTP headers (including the HTTP post command) simply cause SMTP syntax errors, while the body of the POST command contains SMTP commands. By checking the CLIENT_POST environment variable, you to reject mail sent in this way.


The value of CLIENT_IP with the order of the bytes reversed. Suitable for prepending to ".in−addr.arpa" or an RBL domain to perform a DNS lookup based on IP address.


Contains a fingerprint, abstracting the contents of the initial TCP SYN packet the client sent to establish the TCP connection. The exact contents of SYN packets depends on the operating system and version of the client, and can therefore reveal interesting information about the type of client connecting to your mail server. The format of the fingerprint is:

wwww:ttt:D:ss: OOO

Where the fields are as follows:

the initial TCP window size


the IP ttl of the received packet


the IP "don’t fragment" bit


total size of the SYN packet (including IP header)


a comma-separated list of TCP options, as follows:


NOP option


window scaling option with value nnn


maximum segment size value nnn


Selective ACK OK


timestamp option


timestamp option with value zero


If asmtpd can guess the client’s operating system based on CLIENT_SYNFP , it will set CLIENT_SYNOS to the value of that guess. For example, to greylist mail from Windows machines, you can run:

   match −q "*Windows*" "$CLIENT_SYNOS" && greylist


This variable is not really an avenger variable, as it is only available in bodytest commands. It specifies the number of bytes of message transfered in the SMTP DATA command, but after converting CR NL sequences to NL . Roughly speaking this is how many bytes are in the message including all headers after the X−Avenger:, SPF-Received, or Received: header.


The value of EtcDir from the asmtpd configuration file (or /etc/avenger by default).


When avenger runs on behalf of a user EXT is set to the part of the address that determines the suffix of the rcpt or mail file. For example, suppose Separator is "−" and the recipient is list−subscribe@host, where host is not a virtual domain. If the AliasFile contains:

    list: user−mylist

Then avenger will be run on behalf of "user" (because alias expansion yields user-mylist-subscribe). EXT will be set to mylist-subscribe.

Note that EXT is empty when there is no suffix, and that it is equal to the name of the system file being processed when avenger is run on a system file. Like RECIPIENT , this variable is not set for bodytest commands.


Set to the name of the local host, as specified by the HostName directive in avenger.conf.


This variable is set when the SPF disposition of the sender is fail, or when asmtpd is unable to send a bounce message to the sender address. In either case, Mail Avenger will reject the mail if the script falls through to the default.


A randomly generated string for this message, which can be useful to correlate calls to rcpt scripts with bodytest scripts. Note this is unrelated to the Message-ID header in the message, but does show up in the Received header that Mail Avenger inserts.


IP address of local end of SMTP TCP connection.


TCP port number of local end of SMTP TCP connection. Ordinarily this will be 25.


The envelope recipient of the message. Note that this environment variable is not present for bodytest programs, since such programs may be run on behalf of multiple users.


The domain part of RECIPIENT , folded to lower−case--i.e., host when RECIPIENT is local@host. Not present for bodytest programs, as noted in the description of RECIPIENT .


The local part of RECIPIENT , folded to lower−case--i.e., local when RECIPIENT is local@host. Not present for bodytest programs, as noted in the description of RECIPIENT .


The envolope sender of this mail message (i.e., the argument supplied by the client to the "MAIL FROM:" SMTP command.)


The hostname part of SENDER , converted to lower-case (i.e., host in user@host).


The local part of SENDER , converted to lower-case (i.e., user in user@host).


A list of DNS MX records for SENDER_HOST , if that hostname has any MX records.


For non-empty envelope senders, asmtpd attempts to see if it is possible to deliver bounce messages for the sender. If not, SENDER_BOUNCERES is set to a three-digit SMTP error code. If the first digit is 4, the error was temporary. If the first digit is 5, the error was permanent. Note that failure to accept bounce messages is considered a MAIL_ERROR as described above, and will cause mail to be rejected by default.


The value of Separator from the asmtpd configuration file. There is no default ( SEPARATOR will not be set if no Separator is specified in the configuration file). However, it should be configured for "+" with sendmail and "−" with qmail.



The result of performing an SPF check on the message. Will be one of: none, neutral, pass, fail, softfail, error, or unknown. Note that SPF0 and SPF are synonymous, but SPF is deprecated as a future release of Mail Avenger will make SPF synonymous with SPF1 .


Also the result of performing an SPF check on the message, but returns different names for the results, to be compatible with newer revisions of the SPF protocol specification. The new names are None, Neutral, Pass, Fail, SoftFail, TempError, and PermError.


The explanation string that goes along with a bad SPF status.


If the Mail Avenger has been compiled with support for the STARTTLS command (using the −−enable−ssl option to "configure"), and the client is communicating over SSL/TLS , this variable will contain a textual description of the algorithm.


SSL_CIPHER_BITS contains the number of secret key bits used by the SSL/TLS ciphers. SSL_ALG_BITS is the number of bits used by the algorithm. For example, if you are using 128−bit RC4 with 88 bits sent in cleartext, SSL_CIPHER_BITS will only be 40, since that is the effective security, while SSL_ALG_BITS will be 128.


If the client has successfully authenticated itself using an SSL certificate, SSL_ISSUER will be set to the certificate signer’s common name, while SSL_ISSUER_DN will be set to a compact representation of the signer’s full distinguished name. The full distinguished name is in the form output by the command:

        openssl x509 −noout −issuer −in cert.pem

Note that this variable is mostly useful if the SSLCAcert file you have given to Mail Avenger contains more than one certificate authority, or signs other CA certificates. Mail Avenger will not accept client certificates if it does not recognize the signer of the certificate.


If the client has successfully authenticated itself using an SSL certificate, SSL_SUBJECT will be set to the client’s common name in the certificate, while SSL_SUBJECT_DN will be set to a compact representation of the client’s full distinguished name. The full distinguished name is in the form output by the command:

        openssl x509 −noout −subject −in cert.pem


The version of the SSL/TLS protocol in use.


An mbox "From " line suitable for prepending to the message before passing the message to a delivery program. (This is mostly useful for bodytest rules.)


The name of the user under which avenger is running.


avenger is just a simple shell script. You can inspect the file to see what it is doing. Most of the interesting operations happen in either asmtpd, or in external programs spawned from avenger. This section documents the interface between asmtpd and avenger.

avenger inherits a unix-domain socket connected to asmtpd on its standard input and output. It sends commands to asmtpd over this socket, and similarly reads replies from it. In order to avoid mixing messages to and from asmtpd with the output of other programs you run, however, the avenger shell script reorganizes its file descriptors so that all communication to and from asmtpd happens over file descriptor number 3.

Each command consists of a single line, followed by a newline (except the return command, which can optionally take multiple lines). There may or may not be a reply, possibly depending on the outcome of the command. Most replies consist of zero or more lines of the form


VARIABLE is typically a variable name that was supplied as part of the command. The avenger shell script records results by setting the environment variable VARIABLE to value, so that it can be accessed by subsequent lines of the script.

Replies are sent in the order in which the corresponding commands were received. However, asmtpd executes requests asynchronously. Thus, one can perform several concurrent operations (such as DNS requests or SPF tests) by simply writing multiple commands to asmtpd before receiving any of the responses.

The "." command is a no-op, but asmtpd echoes the "." back to avenger as the reply. This allows one to synchronize the avenger process’s state after issuing one or more commands. For example, one might issue several DNS lookups to check various RBLs (real-time blackhole lists), then issue a . command, then wait for replies. When the . comes back, all previous commands will also have completed. The avenger setvars command simply sends a ".", then loops until it reads back the ".", setting variables from any previous commands whose replies it reads in the process.

The following commands are available:


The . command is simply echoed back by asmtpd.

bodytest command

Ends the current avenger script. Specifies that asmtpd should receive the entire body of the message, then run command (under the same user ID as the current avenger script) with the entire mail message as its standard input. asmtpd then replies to the SMTP "DATA" command based on the exit status of command as follows:


If command exits with status 0, asmtpd will reply to the "DATA" command with success ( SMTP code 250), and will pass the message to sendmail (or whatever you have configured as Sendmail in asmtpd.conf) for delivery.


If command exits with status 99, asmtpd will still reply to the "DATA" command with a successful 250 reply code, but will not spool the data. Either command must have done something with the data, or the message will be lost.

100 (also 64, 65, 70, 76, 77, 78, 112)

If command exits with status 100 (or any of the above exit statuses), avenger will reject the mail with a hard SMTP error (code 554). If command wrote output to its standard output, this output will be passed back to the mail client. Otherwise, asmtpd will supply the text "message contents rejected."

111 (or any other exit status)

If command exits with status 111, the result is the same as exit status 100, except that asmtpd will use a temporary error code (451) instead of 554.


If command exits abnormally because of a signal, asmtpd will also use 451, but in this case will not pass the program’s output back to the client. It will instead pass back a description of the problem.

Note that asmtpd can only run one bodytest command per message. If there are multiple recipients of a message, all must run the same bodytest under the same user ID . If two users wish to run different bodytest commands, or even run the same command under different user IDs, asmtpd will defer the second SMTP "RCPT" command with the message:

452 send a separate copy of the message to this user

This will cause the mail client to re-send the message later to the second user. To avoid forcing clients to send multiple copies of messages, you can place bodytest commands in system wide files (such as the default rule file), or use a redirect command to redirect to the AvengerUser, so that commands for multiple users can be run under the AvengerUser user ID .

Note that file descriptor 0 inherited by command is opened for both reading and writing. Thus, it is possible to modify the message before it is spooled by the local MTA . The command edinplace(1) is useful for running messages through spam filters that annotate messages before spooling them.

dns-a VARIABLE domain-name

Requests that asmtpd perform a DNS lookup for A (IPv4 address) records on domain-name. If such an A record exists, the reply is a list of one or more IP addresses:

VARIABLE =IP-address ...

If no such A record exists, the reply is simply:


With the standard avenger script, this sets VARIABLE to the empty string. If there is a temporary error in DNS name resolution, there is no reply, and hence with the default avenger script VARIABLE will remain unset.

When checking such things as RBLs, it is advisable not to reject mail because of a temporary DNS error. You can use the shell construct ${ VARIABLEdefault}$ to return $ VARIABLE when VARIABLE is set, and default when VARIABLE is not set. Similarly ${ VARIABLE +set} returns set if VARIABLE is set, and the empty string otherwise.

For example, if bad−senders.org contained an RBL of undesirable sender hosts:

    echo dns−a BADSENDER "$SENDER_HOST".bad−senders.org >&3
    test −n "$BADSENDER" && reject "$SENDER_HOST is a bad sender"
    test −z "${BADSENDER+set}" \
        && defer "$SENDER_HOST.bad−senders.org: DNS error"

Note that when using the avenger script, there is already a function rbl to check RBLs.

dns-mx VARIABLE domain-name

Similar to dns-a, but looks up MX records. A successful reply is of the form:

VARIABLE =priority−1:host−1 [priority−2:host−2 ...]

Where priority−1 is the MX priority of host−1. As before, an empty string indicates no MX records exist, and no reply indicates an error.

dns-ptr VARIABLE IP-address

Returns a list of verified DNS hostnames for IP-address. As before, an empty string for VARIABLE indicates no PTR records exist, and no reply indicates an error.

dns-txt VARIABLE domain-name

Similar to the other dns commands, but looks up a record of type TXT . If multiple TXT records exist, returns only one. Places some restrictions on the TXT records, for example will not return one that contains a newline character.

netpath VARIABLE IP-address

Maps out the network hops to IP-address (this is similar to the traceroute system utility, but more efficient). The reply is of the form:

VARIABLE =#hops hop1 hop2 ...

#hops is the total number of network hops to IP-address if asmtpd can figure this out. (It won’t always be able to if IP-address is behind a firewall.) If asmtpd cannot figure this out, the value is −1. hop1 and the remaining arguments are the addresses of routers along the way to IP-address.

redirect local

Terminates the current avenger process, and instead processes the mail as though it is being sent to local. This command is only available in "rcpt" mode, as opposed to "mail" mode (in which asmtpd runs avenger to see if it should relay mail for a local user on a non-local client machine).

local can be a local user name, or a local user name followed by the separator character and an extension. The name is mapped using the aliases (specified by AliasFile in asmtpd.conf).

Note that while the AvengerUser user can redirect to other users, ordinary users can only redirect to themselves or the AvengerUser.

return code explanation
code explanation

Specifies the SMTP reponse desired. Also avoids further processing of the message with system-wide default rulesets (as typically happens when avenger simply exits with status 0). code must be a three digit number beginning 2, 4, or 5. (usually 250 for success, 451 to defer mail, and 554 to reject mail).

The first form of this command (with a space between code and explanation) gives a single line explanation along with the result code. In the second form, avenger specifies a multi-line response. In this case all but the last line must contain a between the code and explanation, while the last line must contain a space. (Note that the return keyword only appears on the first line; after starting to issue a return command, no further commands can be issued.)

spf VARIABLE SPF-mechanism ...
VARIABLE SPF-mechanism ...
VARIABLE SPF-mechanism ...

Evaluates the mail client based on SPF mechanisms. It will return:

VARIABLE =disposition

where, for spf0, disposition is one of: none, neutral, pass, fail, softfail, error, or unknown (though the disposition none is actually impossible). For spf1, the equivalent disposition names are None, Neutral, Pass, Fail, SoftFail, TempError, PermError. (Currently spf is a synonym for spf0, but it is recommended that you avoid using spf as in a future release it may become an alias for spf1.)

As an example, suppose that your username is "joe", Separator is "+", and you have subscribed to a number of yahoo mailing lists using email address "joe+yahoo". If spammers started sending mail to "joe+yahoo", you would want to reject all mail to that address except that originating from yahoo’s computers. Yahoo’s computers might correspond to anything ending ".yahoo.com" or sharing a 24−bit IP-address prefix with any of yahoo.com’s MX records. This can be accomplished with the following script in $HOME/.avenger/rcpt+yahoo:

    echo spf YAHOO ptr:yahoo.com mx:yahoo.com/24 −all >&3
    case "$YAHOO" in
        reject "Sorry, this private alias for Yahoo lists only"
        defer "Sorry, temporary DNS error"


If you never use your email address as an envelope sender, you can reject all bounces to that address with these commands in your rcpt file:

    test −z "$SENDER" \
        && reject "<$RECIPIENT> not a valid sender;" \
        " should not receive bounces"

The following script runs spamassassin (a popular spam filter, available from <http://www.spamassassin.org/>) on the body of a message, unless the sender of the message has an SPF disposition of pass or is already going to be rejected by default.

    # The next line immediately falls through to the default reject
    # disposition when mail has an SPF disposition of fail or the
    # sender does not accept bounce messages.
    test "$SPF" = pass \
        || bodytest edinplace −x 111 spamassassin −e 100

The following script immediately accepts any mail from any machine at MIT or NYU (provided MAIL_ERROR is not set), "greylists" machines not in one of those domains, and if the greylist passes, falls through to the the default, system-wide rules:

    spf TRUSTED ptr:nyu.edu ptr:mit.edu ?all
    test pass = "$TRUSTED" && accept Trusted sender OK

The following script rejects mail from clients that have issued an SMTP " POST " command (which doesn’t exist) or used aggressive, premature pipelining of commands. If the client put a space after the colon in the MAIL FROM: or RCPT TO: SMTP commands, it greylists the message using a key that includes the SYN fingerprint and first 24−bits of the IP address. If the SPF disposition of the message is error, it defers the message. If the SPF disposition of the message is softfail or none, it runs the body of the message through spamassassin.

    test −n "$CLIENT_POST" −o −n "$CLIENT_PIPELINING" \
        && reject "no spam please"
    test −n "$CLIENT_COLONSPACE" \
        && greylist "${CLIENT_IP%.*} $CLIENT_SYNFP $SENDER"
    case "$SPF" in
            defer "Temporary error in SPF record processing"
            bodytest edinplace −x 111 spamassassin −e 100

If you set your MACUTIL_SENDER environment variable to be "user+bounce+*@your.host.com" and send mail with macutil −−sendmail, you can create the following rcpt+bounce+default to accept mail only to valid bounce addresses.

    macutil −−check "$SUFFIX" > /dev/null \
        || reject "<$RECIPIENT>.. user unknown"

In conjunction with this script, you may want to reject bounce messages to your regular email addresss with your rcpt script, as described in the first example.

This example is slightly more complicated, and shows how to use a bodytest to reject mail based on message contents. The goal of this set-up is to check each message with the ClamAV anti-virus software (from <http://www.clamav.net/>) and the spamassassin mail filter. If the message contains a virus or is flagged as spam, it should be rejected with an explanation of the problem. We construct a shell script, $HOME/.avenger/body, to run these tests on message bodies. The script can be invoked with the line

bodytest $HOME/.avenger/body

in your $HOME/.avenger/rcpt file. Or, alternatively the script could be configured to run in the system-wide /etc/avenger/default file (in which case you want to make sure that the AvengerUser can write its own home directory, so as to store spamassassin files). The script is as follows:

    out="`clamscan −i −−no−summary −−mbox −  2>&1`"
    if test "$?" = 1; then
        echo This message appears to be infected with a virus
        printf "%s\n" "$out" \
            | sed −e '/Warning:/d' −e 's/^[^:]*: //' | sort −u
        exit 100
    out="`edinplace −x 111 spamassassin −e 100`"
    case "$?" in
            exit 0
            echo Sorry, spamassassin has flagged your message as spam
            while read a b c; do
                test "$a $b" = "Content analysis" && break
            read a
            read a
            read a
            while read a b c; do
                case "$a" in
                    printf "  %s\n" "$c"
                    printf "    %s\n" "$a $b $c"
            exit 100
            if test −n "$out"; then
                echo spamassassin failure:
                printf "%s\n" "$out"
                echo system error in spamassassin
            exit 111

The first half of this script runs the clamscan virus checker, storing the output in variable out. clamscan exits with code 1 when a virus is found, exits 0 on success, and uses other error codes to indicate various system errors. We only want to reject mail if clamscan exits with code 1. When this happens, we take the output of clamscan, format it in a more pleasing way (stripping out warnings), and send it to standard output. An example of an SMTP transaction using this bodytest and detecting a virus will look like this (tested with the special EICAR test string that flags a positive with most virus checkers):

    354 enter mail, end with "." on a line by itself
    Subject: eicar test
    554−This message appears to be infected with a virus
    554 Eicar−Test−Signature FOUND

If the virus check fails, the script runs the message through spamassassin to check for spam. Note that spamassassin modifies the mail message, so that we must run it with edinplace. Note also that clamscan will read to the end of the input file, but this is okay since edinplace rewinds its standard input. We use the −e flag to tell spamassassin to exit 100 on spam. Then, if spamassassin exits 0, we accept the mail. If it exits with anything but 100, something went wrong and we temporarily defer the mail. Note that it might also be possible to accept the mail at this point, but since spamassassin edits the file in place, the message may be truncated if spamassassin exits unexpectedly.

If spamassassin exits 100, we reject the mail. We also report on why spamassassin has rejected the mail. Here again we take advantage of the fact that edinplace rewinds its standard input both before and after processing a message. Because the file descriptor has been rewound, we can start processing the message one line at a time with the shell script. Spamassassin by default (if you have not configred it with "report_safe 0") contains a spam report like this:

 Content analysis details:   (11.7 points, 5.0 required)
  pts rule name        description
 −−−− −−−−−−−−−−−−−−− −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
  1.0 RATWARE_RCVD_AT Bulk email fingerprint (Received @) found
  4.2 X_MESSAGE_INFO  Bulk email fingerprint (X−Message−Info) found
  0.0 MONEY_BACK      BODY: Money back guarantee
  0.5 BIZ_TLD         URI: Contains a URL in the BIZ top−level domain
  0.6 URIBL_SBL       Contains a URL listed in the SBL blocklist
                      [URIs: crocpeptide.biz]
  0.5 URIBL_WS_SURBL  Contains a URL listed in the WS SURBL blocklist
                      [URIs: crocpeptide.biz]

We skip over the headers, and for each result, print it to the SMTP session. Negative/whitelist results (those starting −), we do not report, and comment lines (not starting with a number) we print indented. A typical SMTP session looks like this (using the special GTUBE test line that triggers spam filters):

    354 enter mail, end with "." on a line by itself
    Subject: gtube test
    554−Sorry, spamassassin has flagged your message as spam
    554−  Missing Date: header
    554   BODY: Generic Test for Unsolicited Bulk Email

Here’s an example of how to use SSL client certificates for authentication. If you have a private CA with common name "My CA " that signs the certificates of all your authorized mail clients, you can place the following in /etc/avenger/relay to permit those clients to relay:

    test "My CA" = "$SSL_ISSUER" \
        && accept "Relaying permitted for client $SSL_SUBJECT"
    reject "relaying denied"


/usr/local/libexec/avenger, /etc/avenger/default, $HOME/.avenger/rcpt, $HOME/.avenger/rcpt* $HOME/.avenger/mail, $HOME/.avenger/mail*


dbutil(1), deliver(1), edinplace(1), escape(1), macutil(1), match(1), synos(1), asmtpd.conf(5), asmtpd(8), avenger.local(8)

The Mail Avenger home page: <http://www.mailavenger.org/>.


avenger (and the configuration files it reads) are shell scripts. In a shell script, it is sometimes tempting to use "echo ..." where one should instead use the command "printf '%s\n' ...". (The later just prints its argument to standard output, while the former interprets various "\" escape codes.)

In shell scripts, one must be careful about variables containing shell metacharacters. For example, it is not safe to run something like:

        bodytest "echo $VAR > $PWD/log"

if variable "VAR" has untrusted contents that might contain characters like ">" or ";". The reason is that $VAR will be expanded and sent back to the SMTP server, which will then pass the expansion to the shell to execute the bodytest. ($VAR effectively gets expanded twice.) The escape utility can be used to avoid these problems. For example:

        bodytest echo `escape "$VAR"` ">" $PWD/log

It is easy to forget to call setvars after a dns, rbl, or spf command.


David Mazieres