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Postfix Documentation
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Principles of operation

The before-filter Postfix SMTP server accepts connections from the Internet and does the usual relay access control, SASL authentication, TLS negotiation, RBL lookups, rejecting non-existent sender or recipient addresses, etc. The before-queue filter receives unfiltered mail content from Postfix and does one of the following:

  1. Re-inject the mail back into Postfix via SMTP, perhaps after changing its content and/or destination.

  2. Reject the mail by sending a suitable SMTP status code back to Postfix. Postfix passes the status back to the remote SMTP client. This way, Postfix does not have to send a bounce message.

The after-filter Postfix SMTP server receives mail from the content filter. From then on Postfix processes the mail as usual.

The before-queue content filter described here works just like the after-queue content filter described in the FILTER_README document. In many cases you can use the same software, within the limitations as discussed in the " Pros and Cons" section below.

Postfix Documentation
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