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: 9211B2E4B96C: to=<firstname.lastname@example.org>, relay=mail.example.com[192.168.1.201]: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)
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
resolv.conf, but changing that file to some other IP doesn't generate a true negative.