I have a client that is running Exchange 2003 Sp1 and has 3 main users. 2 of the users can not send emails to the domain kstooldie.com, but one of them can. They are all connected to the domain and exchange server exactly the same way, and all other emails work fine. The 2 users that cannot send to the domain get this error message back...
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<allisonsentry.com #5.5.0 smtp;550 <email@example.com>: Recipient address rejected: No such domain at this location (firstname.lastname@example.org)>