[exim] Route to another host if mailbox doesn't exist.

Folks,

I'm trying to plan the migration of a whole domain's email away from
google over to exim and would appreciate any feedback from the list as
to the best way to achieve this and what tools are available.
I'm aware of imapsync and have used it frequently to take backups of
mailboxes before deletion but would like a nicer way, if possible, to
migrate users with minimal downtime for them.

Regards
Steve.