I have this peculiar problem, and I've spent hours looking for a solution. I'm sure its very simple, yet it escapes me.
I have a server, lets call it server1.com. It has ispmanager and postfix installed. Postfix is configured through ispmanger to process mails from domains server2.com, server3.com, but not server1.com. Mail for server1.com is served by third-party mail system (mx is set correctly to that 3rd party). Now what really happens is that sending & receiving mails to and from server2 or server3 - works, but if I sent mail from server2.com to server1.com - I get this error instantly (in Thunderbird):
An error occurred while sending mail. The mail server responded: 5.1.1 <firstname.lastname@example.org>: Recipient address rejected: User unknown in local recipient table. Please check the message recipient email@example.com and try again.
550 5.1.1 <firstname.lastname@example.org>: Recipient address rejected: User unknown in local recipient table; from=<email@example.com> to=<firstname.lastname@example.org> proto=ESMTP helo=<[X.X.X.X]>
Now if I set in main.cf:
mydestination = localhost.$mydomain, localhost (eg. removing $myhostname)
E-mail goes away, but all "normal" mails to & from server2 - are not coming.
This has something to do with relay settings?
Will be much obliged for any advice.