I have Exchange 2003 and Outlook 2003 as the mail client -- one of my users sent out an e-mail and it was returned as undeliverable with the following message:
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.<mail.mydomain.com #5.5.0 smtp;550 recipient rejected
I'm not sure why this was returned - any help would be appreciated. Thank you.