Since upgrading to GW8 1 week ago, a user is having issues with sending mail to certain domains. This is the latest:
Date: 4 July 2009 5:09:09 PM
To: matthewp.OAK_PO.OAK_DOM
Subject: Message Status - Undeliverable

The message that you sent was undeliverable to the following: (550 Sender verify failed)

Can anyone explain why now is this showing up? He has no issues sending to other domains.