I have SBS2003 server installed and receive this error when sending from and Outlook client.
You do not have permission to send to this recipient. For assistance, contact your system administrator.
<SERVER.kass.local #5.7.1 smtp;550 5.7.1 <email@example.com>... Relaying denied. Proper authentication required.>
The email client has both a Exchange Server setup and a POP3 setup in their client. If Exchange server is selected as the account to send this error is received. If the POP3 account is selected it goes out fine.
Any help would be appreciated.