Moving Some Domains from Exchange 2010 to Office 365

We have an Exchange 2010 server with several email domains. We're moving 2 of those domains to Office 365. What do we need to do to maintain mail flow from the remaining Exchange users to the migrated 365 users?
TechGuy_007Asked:
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TechGuy_007Connect With a Mentor Author Commented:
We ended up deleting mailboxes and creating mail users connected to each user's AD profile, then entered the 365 onmicrosoft email address in for the mail user. Workaround, but it works.
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Ibrahim BennaTechnology LeadCommented:
Will you still have users/mailboxes that will have addresses from these 2 domains? I assume you are planning on setting up a hybrid configuration in order to move mailboxes over?
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TechGuy_007Author Commented:
I'm sorry, I don't understand your first question. Let's say we have 5 domains on the onsite Exchange server, and we're moving 2 of those domains to 365 entirely - they will not be using the onsite Exchange server at all anymore. We want to make sure that users from the remaining 3 domains don't have issues emailing anyone from the 2 migrated domains. We plan to remove the domains from the accepted domains in the onsite server - just not sure if we need to update x500 addresses / how to do that.

The hybrid configuration isn't working. 365 support could not tell us why or how to fix it, so we're manually creating accounts and importing PSTs to 365 (there are only about 20 users so not too painful).
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IvanSystem EngineerCommented:
Hi,

for that configuration to work, you would need to:
1. Register those 2 domains to office365.
2. Create user accounts and assign license, so that they have mailbox created.
3. On o365 Exchange, configure those 2 domains as authoritative, create email address policy.
4. Test to see if email is working between those o365 users, via webmail. It should work internally.
5. Reconfigure MX records for those 2 domains to point to o365.
6. On local Exchange, remove accepted domain and email address policy for those users, so that other onsite Exchange users will now use public MX and therefor route email for those 2 domains to o365.
7. Reconfigure Outlook clients to connect to o365.

There will probably be some down time in steps 5-7, so maybe you can do this off work hours or during weekend.

Regards,
Ivan.
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Todd NelsonSystems EngineerCommented:
If the hybrid config is not working, as you say, are you still using AD synchronization?

The unfortunate part is if you are still using on premises mailboxes for the domains you added to O365 you will have to manually set the TargetAddress attribute in AD (if the hybrid config did not work) so Exchange knows that those specific mailboxes may reside elsewhere and forward the messages appropriately to O365.  Can be tricky but doable.

Personally, I would make every effort to get the hybrid working appropriately to make administration relatively easier.
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TechGuy_007Author Commented:
Sought outside help from Microsoft and solved issue outside of EE suggestions.
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