[Quick Fix] How To Solve Mailbox Exists On-premise And Office 365

The methods in this article help you to solve duplicate mailboxes in on-premises and office 365.

Here’s a sample customer who has an on-premise mailbox.

The same user also has a mailbox in cloud.

There are a couple of options to get rid of the issue. They are listed below.

Option 1: Remove Exchange Online License

• Remove the Exchange online license.
• The object will appear as mail user after the Exchange Online license is removed.

• Now, the mailbox available in on-premise will be migrated to Exchange online and assign the necessary license.
• Once the mailbox appears ok, perform the mailbox restore request to merge items from the soft-deleted mailbox to the active mailbox.

Option 2: Delete the Azure Active Directory account

• As you are aware, exchange online mailbox is associated to an Azure Active Directory account.
• When the Azure Active Directory account is removed, the mailbox in cloud moved to soft-deleted and not appear as active.

• After sync, the on-premise account will sync to Azure Active Directory again as a mail user.

• The mailbox needs to be migrated to Exchange Online and assign the appropriate license.
• Once it is done, perform the mailbox restore request to merge data from the soft-deleted mailbox to the active mailbox.

For more information on mailbox exists on-premise and office 365, check Microsoft website.

Want to improve your Exchange Online experience for better productivity? Check out the tips and tricks mentioned here.