Есть связка: postfix + amavisd-new + spamassassin.

В /etc/amavisd.conf:

$sa_spam_subject_tag = '***SPAM*** ';
$sa_tag_level_deflt  = -9999.0;  # add spam info headers if at, or above that level
$sa_tag2_level_deflt = 6.0;  # add 'spam detected' headers at that level
$sa_kill_level_deflt = 6.9;  # triggers spam evasive actions (e.g. blocks mail)
$sa_dsn_cutoff_level = 10;   # spam level beyond which a DSN is not sent
$sa_quarantine_cutoff_level = 25; # spam level beyond which quarantine is off

Письма  явно проверяются на спам, в теле письма имеем:
X-Spam-Flag: YES
X-Spam-Score: 7.534
X-Spam-Level: *******
X-Spam-Status: Yes, score=7.534 tag=-9999 tag2=6 kill=6.9 tests=[BAYES_99=4.5, RCVD_IN_XBL=3.033, STOX_REPLY_TYPE=0.001]

Но subject упорно не меняется... Подскажите куда копать?!