Ok. I added 2003 Exchange to my 5.5 ORG. I moved several users to Exchange 2003 for testing purposes. Here is the problem. I can't send mail to some, but can send mail to others on the 5.5 server. I get an undeliverable response saying that "The e-mail address could not be found. Perhaps the recipient moved to a different e-mail organization, or there was a mistake in the address. Check the address and try again." I don't understand why mail for some users goes through and for others it does not. PLEASE HELP.