Thanks for your response, Greg.

Unfortunately, Active Directory does not contain a
"RoutingAddress"-type attribute, only the user's HomeMTA (and their
username is not part of that attribute, it's a separate one). We really
do not want to have to customise the AD to add an additional attribute
of the form "u...@exch1.example.com" (with thousands of users and
dozens of mailbox moves a week, it'd be a nightmare to administer).

This is the problem with Exchange in that users belong to the Exchange
Organisation (or AD) and not to a specific server as a local user.

I was hoping that with transport maps we could specify a
"next-hop"-type thing where u...@example.com somehow automagically
knows its mail needs to be delivered to exch1 without having to provide
a fully qualified u...@exch1 address. Well, it was a nice idea while it
lasted. :-)