We have an Exchange 2003 mailbox that the user is connecting to through a POP3 Outlook 2007 client and pulling his and pushing his email, and he's getting various results with it. For example, some email comes back saying "550 5.7.1 Unable to relay for [address]". Another email came back saying "Email account doesn't exist at organization", when I know that they do, and have sent to the very same recipient from other corporate mailboxes (not via POP). But - some email works, and sends successfully from this mailbox. So - not sure what's wrong with the configuration, and could use some advice.