Postfix – Forcing the From Address When Relaying Over SMTP

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I'm trying to get email reports from our AWS EC2 instances. We're using Exchange Online (part of Microsoft Online Services). I've setup a user account specifically for SMTP relaying, and I've setup Postfix to meet all the requirements to relay messages through this server. However, Exchange Online's SMTP server will reject messages unless the From address exactly matches the authentication address (the error message is 550 5.7.1 Client does not have permissions to send as this sender).

With careful configuration, I can setup my services to send as this user. But I'm not a huge fan of being careful – I'd rather have postfix force the issue. Is there a way to do this?

Best Answer

This is how to really do it in postfix.

This config changes sender addresses from both local originated, and relayed SMTP mail traffic:

/etc/postfix/main.cf:

sender_canonical_classes = envelope_sender, header_sender
sender_canonical_maps =  regexp:/etc/postfix/sender_canonical_maps
smtp_header_checks = regexp:/etc/postfix/header_check

Rewrite envelope address from email originating from the server itself

/etc/postfix/sender_canonical_maps:

/.+/    newsender@address.com

Rewrite from address in SMTP relayed e-mail

/etc/postfix/header_check:

/From:.*/ REPLACE From: newsender@address.com

Thats very useful if you're for instance using a local relay smtp server which is used by all your multifunctionals and several applications.

If you use Office 365 SMTP server, any mail with a different sender address than the email from the authenticated user itself will simply be denied. The above config prevents this.

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