Link Exchange Online Mailbox with Exchange On-premise

One of my customer have a hybrid exchange environment.

For a new user needs to create AD account and Mailbox on Exchange on-premise.

After creating mailbox and if is needed (depends on Company Police), user mailbox moving to the Exchange Online (Office 365).

It is important to be sure that the right routing address is set, user listed in Address Book and all works fine.

In this case some mailboxes was created wrong.

That means, that there was a AD user on-premise and mailbox was created directly in the Exchange Online.

 

 

 

When on-premise user tried to send message to user whose mailbox was created in Exchange Online directly, he got message:

 

 

This happened because there is no mailuser on the Exchange on-premise.

 

To fix this issue open a remote PowerShell connection to the Office 365 Tenant and run the following cmdlet:

[PS]Get-Mailbox user | fl ExchangeGUID

 

 

Copy the displayed GUID of the Exchange Online.

On Exchange on-premise start the Exchange Management Shell.

 

Run the following command:

 

[PS]Enable-RemoteMailbox <user> -RemoteRoutingAddress <user>@domain.onmicrosoft.com

 

We create a mail-enabled object in Exchange on-premise and configure the target Address for remote routing and coexistence.

Now we are able to see in the Exchange Admin Center (Exchange Console)

The next step, we need to change the GUID for the enabled mailuser.
On Exchange on-premise in the Exchange Management Shell

[PS]Set-RemoteMailbox <user> -ExchangeGUID <ExchangeGUID>

 

 

This command set in Exchange on-premise ExchangeGuid so it matches Exchange Online (Office 365).

 

The last step, start AD Sync on the AAD Connect server:

 

[PS]Start-ADSyncSyncCycle -PolicyType Delta

 

 

 

 

I hope this solution will help you to fix this kind of issues. I think it is more smoother than to delete the mailbox and recreate it.