Has anyone ever determined the cause and/or a solution to this? I rarely use webmail and it was a client that brought this to my attention. SquirrelMail works great in sending email to the same domain (that the user is logged in to) or another domain on the same server. Uebimiau does allow the email to be sent if the domain is located on a different server, but not to local domains. Some kind of Uebimiau bug?
011907 - Apparently the ORDB info in exim.conf seems to have cuased this, the problem ceased after removing the ORDB info and restarting exim.