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What I want to do is the following: A user from Company X is no longer working there. So, the account can be deleted, but the e-mail should stay accessible for colleagues for a while. I want to convert the mailbox to a shared mailbox.

But, local AD is synced to Azure AD with AD connect. How to convert this users mailbox to a shared (non-synced) one? We do not have Exchange on-premises.

I guess I should do this, but I am not sure:

  • Use Exchange admin panel to convert user to a shared mailbox.
  • Assign permissions to access the shared mailbox.
  • Remove Office365 license.
  • Delete user from local AD
  • Wait for AD connect to do a sync

Please confirm this would work. I do not want to risk that the shared mailbox get's deleted because I removed the user from local AD

EDIT: Just tried the following:

  • Deleted user from local AD
  • Forced sync with AD connect
  • Go to Office admin panel > deleted users
  • Recover the user. I now see it as "cloud" account.
  • When user is recovered, use "convert to shared mailbox".
  • Remove licenses from user.
  • It now is a shared mailbox, not using licensed, and not synced from local AD. Just what we wanted.
  • HOWEVER: As soon as AD connect runs it sync again, the restored user (which is a shared mailbox now) is deleted again.

How to prevent this?

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Your plan was valid, apart the step where you delete user account in local AD. You should keep it disabled as long as you are using shared mailbox.

Where are several methods to restore user in local AD. Check https://technet.microsoft.com/en-us/library/dd379509(v=ws.10).aspx

So your plan now is:

  • Undelete user in local AD
  • Run AD Connect sync
  • Make sure mailbox at Office 365 is configured as shared
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In my account, for some reason the option to convert to a shared mailbox was only available for cloud accounts, not ones synced with AD. So I disabled the user account in local AD, forced a sync with Azure AD, then used Powershell to change the type to a shared mailbox:

Set-Mailbox -Identity user@domain.com -Type Shared

This was as per the page Convert a mailbox from MS docs. Worked like a charm.

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This is difficult in a hybrid environment even if you are running an on-premises Exchange server: you can't convert a remote mailbox from user to shared, but you can't also simply change the mailbox type in Exchange Online, because after a while ADConnect will resync those AD attributes that mark it as a user mailbox.

https://docs.microsoft.com/en-us/office365/admin/email/convert-user-mailbox-to-shared-mailbox

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.

Here's why: if you convert the mailbox in the cloud, it can get converted, but on-premises still thinks the mailbox is the user mailbox, because the new reality does not sync back to on-premises.

Usually this is not a problem, but there are some scenarios where the attributes on-premises (which think that the mailbox is the user mailbox) can overwrite the new cloud version of those attributes, and as a result the mailbox might convert back. This is a problem because user mailboxes require licenses or they are soft deleted after 30 days!

There is a (not officially supported) workaround, which requires manually editing some AD attributes for the user object associated with the mailbox:

http://jetzemellema.blogspot.com/2016/02/convert-user-mailbox-to-shared-in.html

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