Migrate on-premise mailboxes to existant online users

New Contributor


We have to migrate on-premise Exchange 2010 mailboxes to Exchange Online, but the problem is that the customer already has created cloud users with mailboxes using the free teams explanatory license. Now, they are going to upgrade the license buying M365 business standard plan.

Is possible to merge the on-premise mailboxes with the already created Exchange Online mailboxes?

Exchange online mailboxes are almost empty, they only have the meetings they have arranged using Microsoft Teams, so those mailboxes are not important for the customer, but they want to keep onedrive and Teams data. Is possible to remove online mailboxes without deleting cloud users before the mailboxes migration? or What is the right way to do it?

5 Replies
best response confirmed by cruizmct (New Contributor)

You can remove the Exchange license and wait for the mailbox to be deprovisioned. And follow the steps here:

@cruizmct I'm happy to see somebody else is facing what I've been facing repeatedly with customers since COVID-19 (when everybody jumped on Teams quickly).


Here's an Exchange Team blog post about the topic of having a mailbox in both on-premises and in EXO at the same time:


The recommended solution from Microsoft (or even better, the Exchange team) is to blow away the on-premises mailbox and then re-enable the user as a Remote Mailbox, then perform New-MailboxRestoreRequest from EXO PowerShell, restoring the soft-deleted on-premises mailbox directly into the live EXO mailbox.  The main caveat is users will need to create new Outlook profiles because they won't maintain their original on-premises mailbox's ExchangeGuid through this process.  But the big benefit is that they won't lose any of their Teams, SharePoint Online, OneDrive, etc. metadata.


If you go this route, you'll find that the New-MailboxRestoreRequest's end in "FailedOther".  Fortunately I've been through this many times now and have confirmed with the Exchange team and MS Support that the failure status is OK, as long as the Copy Progress shows the correct number of items having been copied.  I just (last Friday and over the weekend) had my Pull Request merged into the MS Docs page for this procedure, which explains this in more detail:

@Vasil Michev 

Thanks Vasil, I was doing a test lab for these situation, having mailboxes before the migration because of the use of the Teams Exploratory free subscription the a lot of customer choose to use. 

I followed the article you gave me and the command worked really fine, I could do the mailbox migration without any issue.

But know, I'm having another problem that I do not know if is because the fact that there were mailboxes created in both sides, on-premise and cloud. I cannot send emails to on-premise mailboxes from this migrated mailbox, I'm receiving the error 550 5.1.10 RESOLVER.ADR.RecipientNotFound.

First, I realized that connectors were not created by HCW using minimal hybrid configuration, but when I rerun it choosing the full hybrid option, the connectors were created. And I beleived that that was the reason, but no, i'm still receiving the same 550 error trying to send an email to an on-premise mailbox, and the same error when I want to validate the connector.

@Vasil Michev 

I have an advance, I could validate the connector and send emails to on-premise mailboxes.

I was checking accepted domains configuration and I saw that the public domain was configured as Authoritative in Exchange Online console, I changed it to Internal redirection, and after that I could send emails to the on-premise mailboxes.

Because of that I'm wondering why HCW configured the accepted domain of Exchange Online as Authoritative.