Convert a user mailbox to a shared mailbox
When you convert a user's mailbox to a shared mailbox, all of the existing email and calendar is retained. Only now it's in a shared mailbox where several people will be able to access it instead of one person. At a later date, you can convert a shared mailbox back to a user (private) mailbox.
Here are some really important things that you need to know:
The user mailbox you're converting needs a license assigned to it before you convert it to a shared mailbox. Otherwise, you won't see the option to convert the mailbox. If you've removed the license, add it back so you can convert the mailbox. After converting the mailbox to a shared one, you can remove the license from the user's account.
Shared mailboxes can have up to 50 GB of data without a license assigned to them. To hold more data than that, you need a license assigned to it. You may need to delete a bunch of large emails (say, ones with attachments) from the shared mailbox to shrink it down so you can remove the license.
Don't delete the old user's account. That's required to anchor the shared mailbox. If you've already deleted the user account, see Convert the mailbox of a deleted user.
The rules are intact after the mailbox is converted to a shared mailbox.
Use the Exchange admin center to convert a mailbox
Go to the Exchange admin center.
Select Recipients > Mailboxes.
Select the user mailbox. Under Convert to Shared Mailbox, select Convert.
If the mailbox is smaller than 50 GB, you can remove the license from the user, and stop paying for it. Don't delete the user's account. The shared mailbox needs it there as an anchor. If you are converting the mailbox of an employee that is leaving your organization, you should take additional steps to make sure that they cannot log in anymore. Please see Remove a former employee from Microsoft 365.
It's not required to reset the user's password during mailbox conversion. However, if the password is not reset, the original username and password continue to work after the mailbox conversion is finished.
Shared mailboxes don’t require a separate license. However, if you want to enable In-Place Archive or put an In-Place Hold or a Litigation Hold on a shared mailbox, you must assign an Exchange Online Plan 1 with Exchange Online Archiving or Exchange Online Plan 2 license to the mailbox.
Convert the mailbox of a deleted user
Let's say you've deleted a user account and now you want to convert their old mailbox to a share mailbox. Here's what you need to do:
Make sure a Microsoft 365 license is assigned to it.
Reset the user's password.
Wait 20-30 minutes for their mailbox to be recreated.
Now follow the instructions on this page to convert their mailbox to a shared mailbox.
After that's done, you can remove the license from the user's mailbox. Don't delete the user's old mailbox. The shared mailbox needs it there as an anchor.
Add members to the shared mailbox.
Convert a shared mailbox back to a user's (private) mailbox
Go to the Exchange admin center.
Select Recipients > Shared.
Select the shared mailbox. Under Convert to Regular Mailbox, select Convert.
Go back to the admin center. Under Users, choose the user account associated with the old shared mailbox. Assign a license to the account, and reset the password.
It will take a few minutes for the mailbox to get set up, but after that, the person who is going to use that account is ready to go. When they sign in, they'll see the email and calendar items that used to be in the shared mailbox.
Convert a user's mailbox in a hybrid environment
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 on-premises attributes (which think that the mailbox is the user mailbox) can overwrite the new cloud versions 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!
We've addressed most of the reasons why this happens but it still CAN happen, although infrequently. It's best to be safe and move the mailbox back to on-premises, convert it, and then move the shared mailbox back to the cloud. This recommended solution is not in violation of the licensing agreement for hybrid environments because the existence of the user mailbox on-premises is only temporary. You would be in violation of your license if you maintained the user mailbox or shared mailbox in your on-premises organization and did not move it back to the cloud.
If you are a member of the Organization Management or Recipient Management role group, you can use the Exchange Management shell to change a user mailbox to a shared mailbox on-premises. For example,
Set-Mailbox -Identity firstname.lastname@example.org -Type Shared.
See the workaround in this support solution for instances when shared mailboxes are unexpectedly converted to user mailboxes.