update: stopped all bes services on original bes.
Installed to a completely new server:
Installed ESM, plus SP2, plus updates. Set BESAdmin as local administrator
Installed BES 4.1.3 (logged in a BESadmin), using MSDE and local database.
Recreated BES config from scratch (I had copies of all the keys and codes handy)
Set MAPI profile and it validated immediately!
Added myself to the BES server, activated immediately
All emails, events and contacts have been sync'd
Tried to send an email from the blackberry and it failed with a red cross
Eventlog on the new BES server says: {user@email.address} Send() failed: ERR_SUBMIT_MAIL, Tag=31
Event ID: 20000
I'm racking my brains trying to think of what I've missed and am coming up blank.