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heavy-d asked ·

Office 365 migration with existing users

I have a customer who has been using Office 365 for the apps but not using the email portion. They now want to migrate thier email that is currently hosted on a third party hosted exchange 2016 server. I have done many migrations but all were without existing users in Office 365. My concern is how the current users will be affected. Will the current user accounts be altered or affected in any way using the Office 365 migration tool?
Thank you

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Hi @heavy-d

Any update here?

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joyceshen-MSFT avatar image
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joyceshen-MSFT answered ·

Hi @heavy-d

Did you deploy a hybrid environemnt with the on-premise Exchange server? Whether the user is assigned an o365 license?

What migration method are you going to use?

The easiest way is to let the user export the on-premise mails through outlook then import them to the mailbox in cloud, change the MX record to make new mails coming to cloud.

You may also refer to the related thread here discussed the similar issue, in a hybrid environmen if you have assigned a license to the user which lead the user has an o365 maibox already : Exchange 365 Migration - Single user, dual mailboxes

Backing up the users 365 mailbox to PST
Revoking the 365 license (which deletes the 365 mailbox)
Forcing an Azure AD sync
Migrating the on-site mailbox to Exchange 365
Re-assigning the license
Importing mails from the PST back into the 365 mailbox again

For more information about How to merge an Office 365 account with an on-premises AD account after hybrid configuration?


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heavy-d answered ·

It is not a hybrid environment. Users do have Office 365 licenses already assigned. The complication comes in that only some users have accounts in Office 365 and only some of those accounts have plans that have a mailbox. Some have office 365 application plans only.
Would this work if I did it this way?
Do a cutover migration
1. The users that do not exist in Office 365 will migrate user and mailbox
2. The users that exist will error out and not migrate and the office 365 user will be unaffected
3. The users that did not migrate I can do a pst export and import
Please let me know if the above is all correct.
Thank you



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Hi @heavy-d

Yes, you could perform the steps above, and for the existing users, you could hide them from the address list on the on-premise server, then they will not list in the migration batch which may cause error.

If there are certain number of existing users, you may also consider using IMAP migration.

You can use the Internet Message Access Protocol (IMAP) to migrate user email from Gmail, Exchange, Outlook.com, and other email systems that support IMAP migration. When you migrate the user's email by using IMAP migration, only the items in the users' inbox or other mail folders are migrated. Contacts, calendar items, and tasks can't be migrated with IMAP, but they can be by a user.

IMAP migration also doesn't create mailboxes in Microsoft 365 or Office 365. You'll have to create a mailbox for each user before you migrate their email.


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Hi @heavy-d

Have you successfully migrated the users?

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heavy-d avatar image heavy-d joyceshen-MSFT ·

I have done some testing and will follow the plan I mentioned above. I have not finished the migration but is in progress. Thank you.

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heavy-d answered ·

I don't see a button to mark your response as the answer so I will just mark one of your previous posts. Thanks again.

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