maybe it helps somebody, but i had a problem with almoust perfectly working mail server.

The server works perfectly all the way up to one point. I can receive e-mails just fine, and same with sending, as long as it's directly from the server. I have roundcube setup on it for webmail, and my clients are happy with it, but they want the ability to use their mail programs (outlook, evolution, thunderbird, etc.).

I tried find solutinon from this forum but i did not find anything.
And my solution was after removing reject_sender_login_mismatch from /postfix/main.cf