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We have about 100 users using POP mail accounts with an externally hosted mail provider with our own company domain name Name@xyzinc.com. I would like to move all of the users over to Office 365 and understand that once we reconfigure the MX record to point to Office 365, the users will all be using 365.
The problem with the above migration procedure is the fact that we (IT) needs to touch everyone's PC to import pst files, drop the POP account and add the Exchange account, something which will take considerable time across our multiple locations. We cannot just change the MX record as days will go by without mail for some.
The solution as I see best from this forum is a solution post titled 'Cloud and POP Coexistence'. This procedure captures a procedure that allows the existence of both Office and POP accounts. As accounts are migrated, the POP account still exists, but points incoming mail over to Name@xyzinc.onmicrosoft.com until all accounts are migrated.
The key to outgoing mail on 365 is that we are adding xyzinc.com as an alternate domain name in 365, but the MX record still points to the POP server. When users on 365 compose email, the reply to address will be xyzinc.com and not xyzinc.onmicrosoft.com.
Once we have completed the migration of all POP mail users, we can then configure the MX record to point to Office 365 from the POP server, and then stop the POP service.
My question to you is whether this process looks good and will provide us with a successful migration. I have to make absolutely sure that we don't interrupt the POP service during the migration and interrupt our companies mail service of the not migrated users.
I have included the details from the post titled 'Cloud and POP Coexistence'.
The situation you're describing is somewhat unusual but it should be possible. It wouldn't be a supported scenario as we have a very specific set of coexistence scenarios, but as it's simple it should be something that will be easy for you to setup and maintain.
In general, co-existence is at its root about mailflow. So we should address it in terms of setting up a mailflow that will work for all of your scenarios. And I'm using the term on-prem here in a generic manner. If your POP solution is Hosted, just pretend I'm saying Hosted.
1) Cloud to On-Prem
This configuration is unchanged. All email from the cloud will keep being routed to your on-prem POP solution.
2) Cloud to Office 365
Instead of pointing your MX records to Office 365, you'll need to setup forwarding rules to send email from the POP account up to the Office 365 account for each user that you've moved onto Office 365.
3) On-Prem to Office 365
This will already be handled by the forwarding rules setup in step #2.
4) Office 365 to On-Prem
You'll set your domain in the Office 365 Exchange Control Panel from Hosted to Shared. That means if Exchange Online can't find a user in your Office 365 user list, it will automatically send it out to the cloud to look up the MX record, which will put it right back up to Step #1.
In a very general way, that's how your configuration would work. Again, this would be unsupported, so we couldn't provide support for issues that arise from this configuration, but it should work without too much trouble.
It's all good, I have it all figured out.