Is it necessary to add all on-prem exchange accepted domains to exchange online before migrating any user for mailflow to work?

Hello Experts. In our env we have Exchange 2013 CU19 enterprise, with AD2012 R2. we are looking into migrating email to O365. we have multiple companies under us, all in one domain forest, with more then 30 accepted domains in on-prem exchange. We would like all mailflow to go through our on-prem environment. I'm looking through the connectors available in O365, and for inbound to O365 connector, i see the attached message. this seems to be only the case for emails coming inbound to o365 from our on-prem servers. It will be difficult for us to add all of our domains to O365 before we migrate any user since we have so many. our plan was to add domains one by one into O365 as we migrate that domains users aswell to O365.

but it sounds as though, then any emails that are sent from other on-prem domains that are not added to O365, that mail will fail to be delivered to the O365 migrated user. is there any way of getting around this limitation. I guess one thing we can do is to point the mx record to O365 as well for the domains whose users will be in O365, and remove that domain from the accepted domain from on-prem exchange, create a connector in O365 to accept emails from the internet. but with this, will the Free/Busy still function and will we still have the unified GAL between on-prem exchange and O365 users? And will we still be able to manage the user exchange accounts from on-prem exchange server?

Please let me know. Thanks in advance
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Newguy 123Asked:
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timgreen7077Exchange EngineerCommented:
even though you have multiple domains and emails domains, I'm assuming you only have 1 exchange environment servicing your different domains. if that this the case, you will want to setup a hybrid environment between your exchange on-prem and O365, and as you move your domains to o365 they will become accepted domains, and that connector will successfully send emails to domain users in O365. you don't need to create a special send connector or anything. domains that are not in o365 will still be able to internally email one another, while any domains in o365 will get internal emails via the connector created with the hybrid setup.
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Todd NelsonSystems EngineerCommented:
One thing to be mindful of is when users have several email addresses based on the multiple email domains you host.

For example, if you currently host the following email domains...

  • domain1.com
  • domain2.com
  • domain3.com
  • domain4.com
  • ...

Then only domain1.com is added to O365 and an attempt is made to move a mailbox with the following email addresses...

  • johndoe@domain1.com
  • johndoe@domain3.com
  • johndoe@domain4.com

The mailbox move will fail because the other domains (used by that specific mailbox) do not exist in O365.

IMO, it is best practices to add all of the domains to O365 prior to moving mailboxes.  Sure, it's a little bit of work, but it's not any more now than it will be later.

Hope that makes sense.

Also, by adding all of the domains before configuring hybrid it will save you from having to re-run the HCW multiple times every time a domain is decided to be added.
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Aaron GuilmetteTechnology Solutions ProfessionalCommented:
For any users that you will migrate to Office 365, you must have an accepted domain verified in Office 365 for those users.  If you don't, you'll receive this error when you attempt to migrate a mailbox for a user whose proxy addresses contain domains not accepted in Office 365:

“You can’t use the domain because it’s not an accepted domain for your organization.”

https://blogs.technet.microsoft.com/undocumentedfeatures/2017/02/15/troubleshooting-mailbox-migration-error-you-cant-use-the-domain-because-its-not-an-accepted-domain-for-your-organization/
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timgreen7077Exchange EngineerCommented:
Answer provided to author. Assigning points and initiating close.
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