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I have a postfix gateway. It only allows emails from the outside to a list of internal users in mydomain.com and it allow emails from my internal mail server to go out to the internet. There is no authentication happening on the gateway.
I would like a simple way to reject any incoming emails with a from address in mydomain.com while still allowing outgoing emails from my internal mail server (preferably only ones with a from address of mydomain.com).
I already have SPF enabled but I can't make it too strict for various reasons unrelated this issue.
Thanks, I think the spf answer does what I want. btw how do I only apply something to port 25 in master.cf? – k-h – 2016-04-30T02:04:51.247
A stock master.cf already has examples for policy specific to port 587 (submission). – user1686 – 2016-04-30T12:54:20.887