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Postfix Documentation
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How Postfix talks to the before-queue content filter

The before-filter Postfix SMTP server connects to the content filter, delivers one message, and disconnects. While sending mail into the content filter, Postfix speaks ESMTP but uses no command pipelining. Postfix generates its own EHLO, XFORWARD (for logging the remote client IP address instead of localhost[127.0.0.1]), DATA and QUIT commands, and forwards unmodified copies of all the MAIL FROM and RCPT TO commands that the before-filter Postfix SMTP server didn't reject itself. Postfix sends no other SMTP commands.

The content filter should accept the same MAIL FROM and RCPT TO command syntax as the before-filter Postfix SMTP server, and should forward the commands without modification to the after-filter SMTP server. If the content filter or after-filter SMTP server does not support all the ESMTP features that the before-filter Postfix SMTP server supports, then the missing features must be turned off in the before-filter Postfix SMTP server with the smtpd_discard_ehlo_keywords parameter.

When the filter rejects content, it should send a negative SMTP response back to the before-filter Postfix SMTP server, and it should abort the connection with the after-filter Postfix SMTP server without completing the SMTP conversation with the after-filter Postfix SMTP server.

Postfix Documentation
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