We have exchange 2003 with server 2003. A user that always was ableto send attchments all of a sudden gets those emails bounced back with a 550 code. I tested it with two emails, one with an attchment and the other without and the one without was delivered, and the one with the attchment was bounced. Same recipient address. I have a hunch that it's something in group policy, but that's only a hunch.