On the same domain, users with mailboxes on an exchange 2003 server cannot send mail to users on exchange 2010 server.
I've tried recreating the client connector. Everything looks ok.
Just get error message:
You do not have permission to send to this recipient.
#5.7.1 smtp;550 5.7.1 Unable to relay
Bounces straight away. no delays.
Users on exchange 2010 can send to users on 2003.