I have an SBS 2003 server running Exchange 2003 SP2. Several days ago we started receiving reports that some users (so far only hotmail and msn that I know of) are unable to send email to our domain. They get a bounced message like the following:
Arrival-Date: Thu, 19 Feb 2009 20:15:42 -0800
Diagnostic-Code: smtp;550 5.7.1 <firstname.lastname@example.org>... Relaying denied. Proper authentication required.
I have tried sending from hotmail as well and get this exact message. Nothing has changed recently in our Exchange configuration. Any ideas?