I didn't provide this detail. My server is fetching mails from ISPs server (using fetchmail), and sending mails through ISPs server.
This is what I get when trying to send mail from gmail to non existing user @mydomain.
Mail Delivery System MAILER-DAEMON@mydomain via "my ISP"
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error.
Reason: Some addresses were rejected by the MDA fetchmail forwards to.
The following address(es) failed:
SMTP error: 550 5.1.1 <user2@mydomain>: Recipient address rejected: User unknown in virtual mailbox table
Final-Recipient: rfc822; user2@mydomain
Last-Attempt-Date: Thu, 08 Nov 2012 14:27:23 +0100 (CET)
Diagnostic-Code: 550 5.1.1 <user2@mydomain>: Recipient address rejected: User unknown in virtual mailbox table