I would also recommend blocking all executable content. To do so, select (highlight) all of the attachment types other than zip files (the last two).
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Click Save.
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====How It Works====
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When receiving an incoming message, the server first tests for RBL and DNSBL listings, if enabled. If the sender is blacklisted, the messages are blocked outright and Spamassassin never sees it.
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With this configuration, the spammiest messages, those marked as 12 or above, will be rejected at the SMTP level. Those spam messages marked between 4 and 12, will be routed to the users' (IMAP) junkmail folder. This is done so the users can check for false-positives...valid messages that were classified as spam by SpamAssassin.
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Users may check their junkmail folders for false-positives via webmail, or, if they are using an IMAP mail client, by simply checking the junkmail folder exposed by their mail client.