I didn't see it mentioned here, there's an official MS fix. This seems to have worked for me so far.
http://www.microsoft.com/downloads/d...playlang=en&tm
I installed this (probably should make sure Outlook isn't running on you phone). After a soft reset, my existing Gmail account started working again (that was getting the send error). Note though that an old email in my Outbox could still not be sent, but new messages seem to work (so far).