question about delivering mail while in "hybrid-mode"

[KTD1]

Hi - We're about to start a migration project from one on-prem exchange server to exchange online. Our outside contractor has decided that "hybrid-mode" was the path to go, so for many months we're going to have users on the on-prem server and in "the cloud". We currently receive mail for only one domain, and have only one on-prem server, so the SMTP route is pretty simple - one entry for "receiving domain" -> one "destination hosts".

Ok, on to the question: Once we enable hybrid-mode, the outside contractor says that mail can only be delivered to one of the two hosts (on-prem or in-cloud). Example: let's say my mailbox has been moved to the cloud. Incoming mail for me would hit the ESA and route to on-prem exchange and then route to the in-cloud mailbox. He states once we hit about 50% of mailboxes moved, we'll flip it and route all mail to the cloud and opposite would be true for someone that hasn't had their mailbox moved yet: incoming mail would hit the ESA and route to in-cloud exchange and then route to the on-prem mailbox.

While that seems like it would work, it seems highly inefficient considering we'll be in hybrid-mode for quite a few months. Is there a way both routes (on-prem and in-cloud) can be active at the same time and the ESA does a lookup to find out where the mailbox is? LDAP maybe?

thanks.

[Ken Stieers]

Caveat, I haven't done it.. but yes, there is an LDAP routing query... the trick being how to get the query to give you something useful/actionable... Basically the query wants to get a mailhost or address out of the LDAP, but I don't think you actually get good info out of AD for that. Its long version not directly the values you need.
You could sort of fudge it.. use an extension attribute, set it to be your internal mail host, then as you migrate people script an update... there'd be an interim while some people's mail goes the long way... but it would work...