We are considering a migration to Microsoft Exchange 2003 server. During the exploration of feasibility etc., we would like to prestage the migration of all 500+ mail accounts using the exchange IMAP migration wizard. The current mail store is approximately 200GB so doing a bulk migration all at once isn't feasible from a training and migration perspective. We would prestage the environment and then as the training group is occuring (20-40 users at a time) rerun the IMAP migration wizard using a date range for emails to merge.
The biggest question that we have at this point is can we have exchange route the messages back to the existing IMAP mail server for the existing accounts that have been prestaged? When a newly migrated user on exchange sends an email to a migrated email group but a user in that email group is still working in the legacy environment (haven't been migrated yet) will exchange attempt to deliver that email locally by default? Is there a way to configure exchange so that it will NOT look to it's own mail stores and by default forward everything out to the existing mailserver where it could then get routed back to exchange through a mailforwarder attribute in their LDAP record?
Thanks in advance for the feedback!