Hey Shane, what server was he using? On our list I'm having problems with several different servers and recieving 550 unknown user errors left and right. I have a list of several servers having these problems, netscape, yahoo, aol, nts.net, etc... what I believe it is, is the servers installing anti-spam software on the network due to client complaints and they're not configuring it right and it's filtering a lot of 'mailing list', 'newsgroup' mail and some of us that use html format in our emails are getting filtered to, so maybe if you try to send him another, make sure it's in plain text format rather than html.
Let me know if that makes any difference, I know this thing is driving me nutzzzzzzzz with our majordomo list and I've been walking people through how to turn off the sever authentication in their email programs so that our mails can get through, it's been a real pain for the last few weeks!
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