This is a useful process a lot of companies follow when an employee departs: Instead of deleting the mailbox, or continue to leave the mailbox in place and pay for licensing, it’s possible to instead set it as a shared mailbox and keep the data there for free.

There are some catches to this, such as the maximum amount of data is 50gb. You also can’t delete the user’s account, but it can be disabled and moved.

Setting the mailbox from User to Shared in Exchange Online is easy (from docs.microsoft.com):

In the admin center, go to the Users > Active users page.

Choose the user whose mailbox you want to convert.

In the right pane, choose Mail. Under More actions, choose Convert to shared mailbox.

…but there’s two tricks I’ve found when doing this in a hybrid environment. First, docs.microsoft.com says to update the status of the mailbox for Exchange On-Premises:

If this shared mailbox is in a hybrid environment, we strongly recommend (almost require!) that you move the user mailbox back to on-premises, convert the user mailbox to a shared mailbox, and then move the shared mailbox back to the cloud.

That’s a tedious process to do just to make it shared. As they point out, you can change some AD attributes locally to get around this, but there’s still some scenarios where it might get set back as a user, have no license, and end up getting deleted.

This other article on support.microsoft.com however, mentions the main way of getting around this: by setting the account’s msExchRemoteRecipientType and msExchRemoteRecipientTypeDetails attributes to the corresponding values that would match it’s state in Exchange Online:

Set-ADUser -Identity ((Get-Recipient PrimarySmtpAddress).samaccountname) -Replace @{msExchRemoteRecipientType=100;msExchRecipientTypeDetails=34359738368}

This 1 line command will set the attributes correctly, you can check via PowerShell or the Exchange Management Console to see that the mailbox will now show as ‘Shared’.

Update 3rd March 2020: Last time I tried the above, it didn’t work. The good news is that as long as you’re on Exchange 2013 CU21 or later and Exchange 2016 CU10 or later, you can just use the command:

Set-RemoteMailbox -Identity user -Type Regular

This fixed the on-premises status of the mailbox, even though I’d already moved it online. So, worth trying first before doing anything, as it should correctly do both if you Thanks Arttu Astila for the tip! /End of update

The other problem I’ve seen is if a mailbox is Unified Messaging (UM) Enabled, and converted to Shared. You’d think that it would either just lose it’s UM status, or let you configure the UM settings after the fact; but neither are correct. If it’s holding onto an extension number as part of UM, even in it’s Shared Mailbox state it will continue to hold it, and block any other account from using the extension in the future.

To get around this issue, the account will need to both be changed back to a user account from shared, and given a license that supports UM. If you try to disable UM on the account with either of these requirements, you’ll see an error like these:

User [email protected] is already disabled for Unified Messaging.

License validation error: the action ‘Disable-UMMailbox’, ‘Identity’, can’t be performed on the user ‘Test User’ with license ‘BPOS_S_Standard’.

With all of the above, changing a user to a departed mailbox in a hybrid environment with Unified Messaging should be:

1. Disable Unified Messaging on the user
2. Set the attributes of the AD account as shared
3. Set the Exchange Online mailbox as shared

It should work well if you do things in the right order, but it’s easy to not be aware of this and get things into a mess.

There’s also the scenario where you might create an account, give it Office 365 licenses and have a mailbox automatically created before you did it on-premises, or used Exchange On-premises to create the mailbox remotely.

You can fix that by using this script from Adaxes (doesn’t need their software!) which will tell on-premises Exchange about the mailbox and create the record.

I’ve come across another blog that goes into some of this http://jetzemellema.blogspot.com/2016/02/convert-user-mailbox-to-shared-in.html but I haven’t needed to change the license status, but it’s worth mentioning in case there’s a scenario you hit where you do.