Hi All
I am currently planning a migration and could really do with some advice/shared experience. I have a bunch of users currently using a hosted exchange enviroment hosted via a 3rd party and Citrix. Migration will involve bringing these users in-house to my exchange 2013/Server 2012R2 where I already have other users working from (They use a different email address for the context of this scenario). Due to resources and to minimise downtime I want to migrate users from hosted to my internal exchange in a couple of phases/chunks over a few weeks.
What I intend to do is keep the MX record pointing to the hosted platform until all users are migrated accross but in the meantime set mailbox forwards on first group of desired users forwarding them to my internal exchange via another temporary domain I have purchased for the migration. I added the currently externally hosted email domain to my authorised domain list in Exchange which worked but of course users already using my exchange who email a user who hasnt been migrated yet get an NDR because their mailbox doesnt exist yet and its trying to route the email internally. I need something where if the mailbox doesnt exist internally to route to the internet/normal MX destintion
I have been reading about External Relay Domains and it sounds like this might be what I am after. Can anyone verify if this is what I am looking for? I want this all to be transparent to all users.
Edit: Reading more it looks like external relay will not work for me as I will have some mailboxes internally matching that domain. internal relay could be the way to go?