A good explanation of this SMTP error can be found here:
http://www.eudora.com/techsupport/kb/1593hq.html
Here is some information regarding SMTP 550 for Exchange Server:
"While there are several causes for this error, the most common one is that there may be no recipient policy for the expected domain in the Exchange 2000 organization or the Exchange 2003 organization."
http://support.microsoft.com/default.aspx?kbid=289553&product=exch2k
