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The easiest way to integrate postfix and spamassassin is to use spamd. First, edit /etc/postfix/master.cf, find the "smtp ... smtpd" line and just add " -o content_filter=spamassassin" to the end of the line.
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spamassassin unix - n n - - pipe user=nobody argv=/path/to/spamc -f -e /path/to/postfix/sendmail -oi -f ${sender} ${recipient}
# make sure it's all on one line or
# start the second line with a whitespace
Then, setup spamd to start with the system, and you are ready to go. If you wish to provide spamassassin preferences, change "user=nobody" to a valid system user (except for root, since Postfix will NOT call external programs as root), and add .spamassassin into their that user's home directory.
Alternatively, a more complex scheme is to integrate it using Amavis, as described in IntegratedInPostfixWithAmavis.
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