Some thoughts on gradual migration:
SmarterMail supports a split domain, where addresses not configured locally are sent elsewhere, in your case to the Exchange 2016 server. I don't think Exchange has an equivalent capability, and even if it does, you need to ensure that invalid addresses do not get caught in an infinite loop.
Suggested strategy for a phased migration, using "example.com" to represent your domain:
- Create an empty example.com domain in SmarterMail, configured as type External, with the Exchange server referenced by IP address. (You must create an admin account, but it does not have to be postmaster or anything else that would conflict with Exchange.)
- Change your incoming route so that all incoming mail is sent to SmarterMail. Verify that all mail is forwarded to Exchange and accepted (if a valid account) or discarded if invalid.
- Also in SmarterMail, create a placeholder domain called example.local, type Local.
- In Exchange, configure a route definition to send messages for example.local to smartermail.
To forward the first user (user1@example.com):
- In SmarterMail, configure an account for user1@example.com
- Also, configure an alias that forwards user1@example.local to user1@example.local
- From an Exchange account, send a test message to user1@example.local and verify the delivery flow all the way to user1@example.com
- In Exchange, configure a contact for user1@example.local so that you can then create a Transport Rule to forward all messages for user1@example1.com to user1@example.local. (Yes, Exchange makes this simple action rather tedious.) Verify the mail flow. Inform User1 that he must now log onto SmarterMail for all new messages.
- The user can migrate his old messages on his own, or use Outlook with two defined accounts to see all his email with one interface, at least until Exchange goes away.
Some users want all their old mail, others do not. So this is an opportunity to purge junk, depending on your corporate records retention policy.
Repeat 999 more times. (Sorry).
Finally, decide what to do with Exchange accounts for terminated employees. Can the messages in Exchange be discarded, or do they have to be archived before Exchange can die?
Once everyone is using SmarterMail for new messages, the
example.com domain can be changed to type Local and the example.local domain can be discarded.
Use or ignore this approach. Just laying out one option.