I have postfix 2.x on Debian 8 setup. I modified main.cf to have smtp_sender_restrictions = permit_mynetworks, reject_non_fqdn_sender, reject_unknown_sender_domain, permit

As of now, for a domain example.com, I can't send mail as user1@example.net but I can send mail as user2@example.com.

How can I restrict users from sending mail as someone else? I don't want bob to act as mary.


By using SMTP authentication. This also means that you cannot accept mail "as is" even from your trusted networks. However, if mail is destined to the domains you serve, you cannot prevent users to send mail as someone else - this flaw is in design of SMTP.

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