Change from local mailbox to Exchange Online mailbox for Disrynched Users RRS feed

  • Question

  • Good morning,

    We're currently migrating a customer off an on-poremises exchange to Office 365.

    Rather than doing a Hybrid we've instead (not my idea, but there was a concern that Migrating mailboxes using a hybrid deployment would take too long):

    -Done a Directory Sync but not included the Mailbox GUID

    -This Syncs the users  to Office 365 and Office 365 doesn't think they have a mailbox, upon being licensed they then get a mailbox on Office 365

    -Used a Third party tool to migrate mailbox contents between the on-premise mailbox to the Exchange Online mailbox in 6 month intervals so the latest 6 months of mail is always and the rest can be backfilled once the users are moved over)

    My question is, what's the process for telling the on-premise user accounts that their mailboxes are the Exchange Online ones once their local ones have been disconnected?

    I'm guessing we'd just overwrite their local mailboxGUID attribute with the GUID of the Exchange online mailbox?

    Edit: If it is as simple as telling them what their mailbox GUID is, would we be able to leave the on-premise mailboxes in place?
    • Edited by Paddy Maxson Thursday, November 28, 2019 8:47 AM
    Thursday, November 28, 2019 8:46 AM

All replies

  • There's no supported method for what you are asking. The on-premises account should still be linked to the cloud mailbox after you disconnect the on-premises mailboxes, but as far as attributes go, best leave them as is.
    Thursday, November 28, 2019 9:13 AM
  • Hi,

    As Vasil L. Michev said, this migration method isn't supported.

    We suggest you create hybrid to migrate mailbox to Exchange. (Hybrid will not consume too much time, in a hybrid environment, you could migrate mailbox gradually, you can also migrate mailboxes at one time)

    If you don't want to create hybrid, you can also migrate mailbox data in the format of Cutover.

    Above migration methods are all suggested and supported by Microsoft.


    In your scenario, when you disable/disconnect mailbox in Exchange on-premises, this action will also effect/sync to Exchange online. So, you need to keep both of Exchange on-premises and Exchange online mailboxes.

    There will exist mail flow issue if you use the same email address from them. You need to keep them in different domain name.

    You can choose the suitable solution based on your needs.


    Kyle Xu

    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact

    Friday, November 29, 2019 7:04 AM
  • Hi,

    I am writing here to confirm with you how thing going now?

    If the above suggestion helps, please be free to mark it as an answer for helping more people.


    Kyle Xu

    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact

    Tuesday, December 3, 2019 8:22 AM