Hey Ger,
I would guess that either the new server does not need to be authenticated against (not very likely) or your client has moved to using MS Exchange and it is set up to authenticate users via NTLM (part of Microsoft's Single Sign-in methodology). This is just a guess though.
You should get in touch with the mail admin to determine what authentication is needed for this new server before figuring out what to do next...
Marty
A client has recently changed Email Server and sending an email now fails with a
....5.7.4 Unrecognized Authentication Error
What is this and how do I rectify ?