Reconnect Previous Mailbox After Hybrid Migration

In the world of email management, maintaining the integrity and accessibility of user mailboxes is paramount. In the Exchange Hybrid Environment, which combines on-premises and Exchange Online (EXO) functionality, occasional hiccups may occur.

One such scenario involves dealing with a user having both an on-premises mailbox and an Exchange Online mailbox because of misconfigurations. Here we deep down into the intricacies of handling the situation and best practices to reconnect the correct mailbox after migration.

Understand the Situation

A user unknowingly ends up with a user mailbox both on-premises and in Exchange Online. This can happen if the user was migrated from on-premises to Exchange Online, but the on-premises mailbox was not deleted. Now, we have to determine which mailbox should prevail, and how to reconnect the correct mailbox post-migration

If you need to reconnect your previous mailbox after migration, you can follow these steps:

  1. In the Exchange Admin Center, go to Recipients > Mailboxes.
  2. Look for the mailbox you want to reconnect.
  3. Click on the Actions menu and select Reconnect Mailbox.
  4. In the Reconnect Mailbox dialog box, choose the On-premises mailbox options.
  5. Enter the name of the on-premises mailbox that you want to reconnect.
  6. Click Reconnect.

This way your previous mailbox will be reconnected to the user mailbox. The contents of the mailbox will be synchronized from on-premises to Exchange Online.

What Happens to Data in an Old Mailbox?

When you reconnect a previous mailbox, the data in the old mailbox is not deleted. However, the old mailbox will be marked as disconnected. This means that the user cannot access the mailbox from Outlook or other mail clients.

If you need to recover the data in an old mailbox, you can follow these steps.

  1. In the Exchange Admin Center, go to Recipients > Mailboxes.
  2. Locate the disconnected mailbox that you want to recover.
  3. Click the Actions menu and choose Recover Disconnected Mailbox.
  4. Select the Recover to Exchange Online option in the Recover Disconnected Mailbox dialog box.
  5. Click Recover.

The data given in the old mailbox will be recovered to a new mailbox in Exchange Online. The user can then access the mailbox from Outlook or other mail clients.


Overall, the best method for reconnecting a previous mailbox after migration is to disable the on-premises mailbox and keep the Exchange Online mailbox. This will ensure that the user’s data is not lost.

If you do need to reconnect the previous mailbox, be sure to export the data from the old mailbox to a PST file before you reconnect it. This will allow you to recover the data if something goes wrong.

Amit Singh

Amit Singh

I am a dynamic Tech Enthusiast, specializing in various Microsoft technologies, including Office 365, Exchange, PowerShell, and Active Directory. I loved to helped customers worldwide design, implement, migrate, and secure Exchange and Office 365 through various roles for different employers, and more recently, as an independent consultant.