On OSX, I don't need to configure postfix at all, I just sudo postfix start and it all works. In the log files, I see

Apr 23 14:04:46 orcrist.local postfix/smtp[19506]: 9211B2E4B96C: to=<me@example.com>, relay=mail.example.com[]:25, delay=0.07, delays=0.03/0.01/0/0.03, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 38571E7C7D3)

How is relay set?

The postconf -d doesn't reveal any parameters defined with that IP, so postfix must determine it at runtime and/or using some other tool. Grepping in /etc doesn't reveal any files with the IP except resolv.conf, but changing that file to some other IP doesn't generate a true negative.


Assuming both parts where you obfuscated the real domain name are actually the same name (example.com and mail.example.com) the likely situation is that you are using internal nameservers (which is that have an MX record defined for whatever example.com really is and it's simply using that record.

That's how SMTP normally works when delivering remotely.

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