Permanently Clear Previous Mailbox Info
Published Jan 17 2018 12:11 PM 319K Views

We are introducing a new parameter that can be called by using the Set-User cmdlet in Exchange Online PowerShell. The feature is focused for customers doing migration of on-premises mailboxes to the cloud and you will be able to use it within three weeks or so (Edit 1/19: we updated this due to slower than expected rollout): Customers who have Hybrid or on-premises environments with AAD Connect / Dir Sync may have faced the following scenario:

  1. User has a mailbox on-premises. Jon is represented as a Mail User in the cloud.
  2. You are synchronizing the on-premises directory to the cloud in preparation to migrate to Exchange Online.
  3. Due to issues with the on-premises sync or due to a configuration problem, the user does not get the ExchangeGUID synchronized from on-premises to the cloud.
  4. If the Exchange GUID is missing from the object in the cloud, assigning an Exchange license to will cause Exchange Online to give the user a mailbox, converting the object from a Mail User to a User Mailbox. (Adding the license is a step required for the migration of the mailbox from on-premises to the cloud.)
  5. The end result is the user that has 2 mailboxes: one on-premises and one in the cloud. This is not good. Mail flow issues will follow.

Those doing these types of migrations will know that the ExchangeGUID value is very important as it helps Exchange Online identify that the user has a mailbox on-premises, and if an Exchange license is assigned in the cloud, a new mailbox should not be created. The immediate fix for this situation is to remove the Exchange License from This will convert the cloud object for Jon back to a Mail User. Mail flow should be restored at this point. The problem now is that you have an “unclean” cloud object for Jon. This is because Exchange online keeps pointers that indicate that there used to be a mailbox in the cloud for this user:

PS C:\WINDOWS\system32> Get-User | Select name,*Recipient*
Name PreviousRecipientTypeDetails RecipientType RecipientTypeDetails
---- ---------------------------- ------------- --------------------
Jon UserMailbox MailUser MailUser

Re-assigning the license after that will always err on the side of caution and Exchange Online will try to re-connect the (duplicate, temporary) mailbox in the cloud (and mailboxes can be reconnected for 30 days). Therefore Jon’s account in the cloud can’t be licensed in preparation for migration. Up to now, one of the few options to fix this problem was to delete *only in the cloud* Jon’s object and re-sync it from on-premises. This would delete from the cloud – but from all workloads, not only Exchange. This is problematic because Jon could have his OneDrive or SharePoint data in the cloud only and deleting his account means that this will be deleted too. If the account is then re-created, Jon and the tenant admin would have to work to recover to his new account all the data he used to have in OneDrive or SharePoint just because Exchange data needed to be “cleaned up”. The new parameter in the user cmdlet will allow tenant admin to clean up Exchange Online Jon’s object without having to delete it. To clean the object, you can run the following command:

PS C:\> Set-User -PermanentlyClearPreviousMailboxInfo
Are you sure you want to perform this action?
Delete all existing information about user “"?. This operation will clear existing values from Previous home MDB and Previous Mailbox GUID of the user. After deletion, reconnecting to the previous mailbox that existed in the cloud will not be possible and any content it had will be unrecoverable PERMANENTLY. Do you want to continue?
Yes  Yes to All  No  No to All  [?] Help (default is "Y"): Y

Executing this leaves you with a clean object that can be re-licensed without causing the 2-mailbox problem. Now you can on-board Jon’s on-premises mailbox following the usual steps. An alternative – a call to support to do the clean-up for you - is also not needed. Remember, cleaning up the user means that the older associated disconnected (duplicate) cloud mailbox is not recoverable. If you want to keep it or be able to check it’s content, we recommend using Soft Deletion or Inactive Mailboxes to keep the mailbox.

Note: This command is expected to be executed when you have an on-premises mailbox and a mailbox in the cloud for the same object due to bad AAD Connect configuration, to clean the object that can be re-licensed. The procedure allows you get out of the dual mailbox state and enable you to re-try on-boarding the mailbox immediately.
If you execute this for a user whose mailbox is cloud only after delicensing the user (and intend to later re-license the same user and expect to have a new clean mailbox for the user) - then this will not happen immediately. To avoid potential loss of mailbox data due to unintended/mistaken execution of the command, we retain the mailbox data for 30 days so that you may recover it. If your intention is to clean-up cloud only mailbox then you may hard delete the user account to re-create a clean mailbox.

Mario Trigueros Solorio

Version history
Last update:
‎Jul 16 2021 08:08 AM
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