I have been changing my mailing addresses from "example.com" to sub domains - say "mail.example.com" to configure no-reply addresses there. I use "sendmail -f noreply@mail.example.com" to send emails making sur ethe Return-Path is correct. My mail delivery chain looks as follows:
SENDING SERVER --> relay_server --> gateway out
For unknown to me reason when my email reaches the gateway that sends out, I see that the "from" address is already altered in the postfix log file to "noreply@example.com". This is also what I see in the detailed message log when it reaches its destination.
I have seen similar topics over here, but I made a step ahead and checked that I use the "-f" sendmail switch.
Any clues?
After the entire weekend of troubleshooting this I eventually found the reason:
http://www.postfix.org/postconf.5.html#masquerade_domains
plus restart of all postfix-related deamons (postfix, dkim and all that) helped.