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Garry ShapeFlag for United States of America

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Exchange Hybrid - create mailboxes in Exchange Online or on-prem then migrate?

Have an Exchange Hybrid environment with AD on-prem, and AD synced to O365 with Azure AD Connect.
All mailboxes get migrated to Exchange Online, so I'm wondering if it's best to create directly in Exchange Online? Or will that create missing-attribute issues and target address issues, especially if MX record mailflow goes to on-prem Exchange first?
I've read that it's safer and less complex to create on-prem then migrate to Exchange Online.
Any pro's/con's to doing it either way?
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timgreen7077

Since you are in a Hybrid environment with AD connect, it is recommended to create the mailbox on-prem and then migrate to O365. That allows the on-prem environment to already be aware of the user object and the mailbox and it will apply the attributes needed for you. Also it makes it 100 times easier if your are attempting to connect to a shared mailbox and view calendars and so forth.
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Michelangelo
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I've seen "Enable-RemoteMailbox" recommended before. Does that work on existing mailboxes? I think I've experimented with it before and ran into issues where it was still an on-prem mailbox yet they had a duplicate mailbox in the cloud.Is there something more to making it work than just using that single cmdlet?
Or perhaps the recommendation to upgrade to latest Exchange? We're on Exchange 2010.
You can use the enable-remotemailbox cmdlet to create a mailbox in O365 for an on-prem user. You are correct you can run into an issue with a duplicate mailbox in the cloud if you are not careful. I found it is so much easier to just create the user and mailbox on-prem, and migrate to O365. There is no mail in the mailbox so it takes seconds and you avoid the troubles of going the other way.
Yeah I have a script that migrated mailboxes every 5 mins if they are on prem and not detected to already be in a migration batch.  
Problem is the completion email says it took 35 mins every time whether it’s 1 new mailbox or 10 I can’t tell if that’s really the case or not.
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