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Planning out a migration for 3 separate Exchange Servers/AD forests with no AD trusts configured; there is site-to-site connectivity by way of IPSec tunnels.

These three mail servers share an email domain by way of Internal Relays (configured for MX lookup, which points to a central ISP-owned Linux mail server with forwarding aliases setup for each recipient's respective Exchange Server).

My question is, can you setup multiple Exchange endpoints for a single Exchange Online tenant? Hybrid Full/Minimal or Cutover? Are there 3rd-party solutions that can do this if not?

gravyface
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This is possible in general, although quite tricky to get right; here's the only offical documentation I was able to find: https://docs.microsoft.com/en-us/previous-versions/exchange-server/exchange-150/jj873754(v=exchg.150). It refers to Exchange 2013 and it's marked as "no longer updated content", but it should still be valid.

Here is another unofficial but more recent article: https://practical365.com/blog/exchange-multi-forest-hybrid-tips-and-tricks/

However, you are using the same SMTP domain in all forests, and this adds a whole lot of issues, not only for mail routing (which probably can still be made to work) but mainly for the Autodiscover service, which can't possibly work for all your users in this scenario, since every Exchange forest doesn't know about users in the other ones; and without a properly working Autodiscover service, lots of things are going to break.

My suggestion would be to perform three cutover migrations in sequence; setting up and running a three-forests hybrid environment where all forests share the same SMTP domain is going to be painful, if it works at all.

Massimo
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  • Was thinking the same thing: logistically, being in 3 places at once, would be near impossible anyways. There will be a net new AD forest setup centrally, which is what we will be running DirSync on for password synchronization, but getting all those profiles flipped over is going to be a helluva lot of boot prints... – gravyface Feb 27 '19 at 21:01
  • Yes, migrating everything to a single forest and *then* going to hybrid is the cleanest solution; I didn't mention it only because I was assuming you didn't want to add *another* forest to this mess :) But if you have enough time and resources, it's better indeed. – Massimo Feb 27 '19 at 22:25