I have a machine with two domain names, say a.com and b.com.
When I use postfix to send something from an address like email@example.com, it replaces it with firstname.lastname@example.org every time. My main.cf file specifies only a.com so I don't know where the b.com is coming from.
Here's the relevant part of maillog... the first line is the intiation of the message and then the 6th line is the one where email@example.com mysteriously pops up.
mymachine sendmail: qA3NGbOR028543: firstname.lastname@example.org, size=792, class=0, nrcpts=1, msgid=<...@a.com>, relay=apache@localhost mymachine postfix/smtpd: connect from b.com[127.0.0.1] mymachine postfix/smtpd: 912F9758031: client=b.com[127.0.0.1] mymachine postfix/cleanup: 912F9758031: message-id=<...@a.com> mymachine sendmail: qA3NGbOR028543: email@example.com, delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30792, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (Ok: queued as 912F9758031) mymachine postfix/qmgr: 912F9758031: from=<firstname.lastname@example.org>, size=1130, nrcpt=1 (queue active)
I'm not sure if it's relevant how I called this, but it's from some PHP mail() and I've ensured the -f flag is passed along. If any more info might help I'd be happy to deliver it!