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Secure Postfix Open Relay

By default a Postfix server is set to the following recipient restrictions:

smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, defer_unauth_destination

What we need to do is set Postfix to reject unauthorized destinations, so they don't fill up our mail queue.

Then add an additional line to reject unknown sender domains so that no other mail can be spoofed.

​smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
smtpd_sender_restrictions = reject_unknown_sender_domain

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