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Postfix: The Definitive Guide by Kyle D. Dent

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Other Considerations

You can run multiple content filters, if necessary, by chaining them. If, for example, you have both an anti-virus and an anti-spam content filter, simply configure the first one to deliver to the next one rather than immediately back to Postfix. The Postfix configuration doesn't have to change from what's presented here. Only the final filter delivers the message back to Postfix.

Be aware of any email address rewriting that occurs before your filter receives a message. When the filter resubmits a message, if the rewritten address isn't in one of the recipient maps, Postfix will reject it. You may have to turn off address rewriting in your normal SMTP server and configure it instead in your SMTP server that accepts messages back from your filter.

Some filters recommend that you configure them to accept mail in front of your normal MTA, and then they pass the messages on to your MTA after processing. You probably do not want to do this. Postfix is specifically designed to accept messages over an unfriendly network. A content filter is specifically designed to deal with processing the contents of messages and probably isn't optimized for dealing with the load and potential hazards of accepting connections from the outside. Likewise some filters want to handle the final delivery of messages without re-injecting them into Postfix. Again, Postfix offers a lot of flexibility and security in dealing with the final disposition of messages that you might lose by delegating ...

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