ZCS uses SpamAssassin to control spam. SpamAssassin uses predefined rules as well as a Bayes database to score messages. Zimbra evaulates spaminess based on percentage. Messages tagged between 33%-75% are considered spam and delivered to the user’s junk folder. Messages tagged above 75% are not sent to the user and are discarded.
When ZCS is installed, the spam/ham cleanup filter is configured on only the first MTA. The ZCS spam training tool, zmtrainsa, is configured to automatically retrieve these messages and train the spam filter. The
zmtrainsa script is enabled through a crontab job to feed mail to the SpamAssassin application, allowing SpamAssassin to ‘learn’ what signs are likely to mean spam or ham. The zmtrainsa script empties these mailboxes each day.
When zmtrainsa runs, these messages are used to teach the spam filter. Make sure you add a large enough sampling of messages to get accurate scores. To determine whether to mark messages as spam at least 200 known spams and 200 known hams must be identified.
The postfix_policy_time_limit key is set because by default the Postfix spawn (8) daemon kills its child process after 1000 seconds. This is too short for a policy daemon that might run as long as an SMTP client is connected to an SMTP process.
For a list of current RBL’s, see the Comparison of DNS blacklists article at http://en.wikipedia.org/wiki/Comparison_of_DNS_blacklists.
If you do not want messages that are identified as whitelisted to be sent to the junk folder, you can configure zimbraSpamWhitelistHeader and
zimbraSpamWhitelistHeaderValue to pass these messages to the user’s mailbox. This global rule is not related to the Zimbra MTA spam filtering rules. Messages are still passed through a user’s filter rules.
