Mail Avenger lets you reject mail based on message contents, before your mail server accepts delivery of mail from a remote client. This has two benefits. First, it can save load on your mail server. Second, and more important, it allows you to reject spam and viruses without silently discarding them, but while also minimizing unwanted bounces to innocent third parties.

Blocking mailing list mail

#!/bin/sh

edinplace
prec="`formail -czx precedence:`"
case "$prec" in
    list|junk|bulk)
	echo You appear to be sending mailing list \
	         mail to an address
	echo that does not accept mailing list \
	         messages.  If you\'d like
	echo to try again, remove the following \
	         header field from the
	echo message before re-sending:
	echo "  Precedence: $prec"
	exit 100
	;;
esac

for field in list-unsubscribe mailing-list
do
    edinplace
    val="`formail -X $field:`"
    if test -n "$val"; then
	echo You appear to be sending mailing list \
                 mail to an address
	echo that does not accept mailing list \
                 messages.  If you\'d like
	echo to try again, remove the following \
                 header field from the
	echo message before re-sending:
	echo " $val"
	exit 100
    fi
done

Next, make the script executable by running the command chmod +x $HOME/.avenger/scripts/no-list. Finally, invoke the script as a bodytest, by placing the following in your $HOME/.avenger/rcpt file:

errcheck
bodytest $PWD/scripts/no-lists

Blocking spam and viruses

Ordinarily, spam and viruses come from forged email addresses. Bouncing such mail therefore causes an innocent third party to receive an unwanted bounce message. On the other hand, since no spam or virus filter is 100% effective, there is always the slim possibility of a "false positive"--some important email getting mischaracterized as spam. Silently discarding such messages can have serious consequences. For that reason, many people bounce spam back to the sender. (Many people also bounce viruses, though this is less defensible, since if you know which virus is infecting a mail message, you probably also know whether that virus typically forges sender addresses or not.)

Mail Avenger offers a good solution to this trade-off. By rejecting mail during the SMTP session, your sever never has to generate a bounce message. If the SMTP client is a legitimate mailer, it will generate a bounce of its own. Thus, you will never silently discard mail. In the common case that the client is a virus or spambot, no bounce will be generated at all.

This example shows how to reject mail with the ClamAV virus checker and SpamAssassin mail filter. Start by creating a an executable script called $HOME/.avenger/scripts/clam-and-spam containing the following:

#!/bin/sh

edinplace
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
fi

out="`edinplace -x 111 spamassassin -e 100`"
case "$?" in
    0)
        exit 0
        ;;
    100)
        echo Sorry, spamassassin has flagged your message \
                 as spam
        while read a b c; do
            test "$a $b" = "Content analysis" && break
        done
        read a
        read a
        read a
        while read a b c; do
            case "$a" in
            "")
                break
                ;;
            -*)
                ;;
            [0-9]*)
                printf "  %s\n" "$c"
                ;;
            *)
                printf "    %s\n" "$a $b $c"
                ;;
            esac
        done
        exit 100
        ;;
    *)
        if test -n "$out"; then
            echo spamassassin failure:
            printf "%s\n" "$out"
        else
            echo system error in spamassassin
        fi
        exit 111
        ;;
esac

Next, make the script executable by running the command chmod +x $HOME/.avenger/scripts/clam-and-spam. Finally, invoke the script as a bodytest, by placing the following in your $HOME/.avenger/rcpt file:

errcheck
bodytest $PWD/scripts/clam-and-spam

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