I have and Exchange Server 2003 fully patched working fine. We have two email domains that the server is responsible for. Address B works perfectly, all mail sent to all recipients is received.
However when mail is sent to the same recipients using email address B some senders get the following bounce:
Your message did not reach some or all of the intended recipients.
Sent: 05/02/2013 09:13
The following recipient(s) cannot be reached:
firstname.lastname@example.org on 05/02/2013 09:10
The e-mail account does not exist at the organization this message was sent to. Check the e-mail address, or contact the recipient directly to find out the correct address.
< box7.911domain.com #5.1.1 SMTP; 550 No Such User Here>
If I send the same message to the same address using a yahoo mail account, it works fine.