Correcting Shared Mailbox provisioning and sizing


We are correcting how shared (and resource) mailboxes are created in Exchange Online.

As per our documentation (here and here), shared and resource mailboxes in Exchange Online should be created with the size of 50GB and if a larger size is desired, a license should be assigned to the mailbox. For some time now, though, while our documentation has (correctly) stated this, the Exchange Online system would, for many of our customers, create both new shared and resource mailboxes with a default size of 100GB even if a license was not assigned. We knew that this was happening, and we were working to address this in our mailbox provisioning system.

This is what is now being corrected to the documented behavior.

The new shared or resource mailbox creation behavior (starting end of July) will by default be 50GB, as per our documentation – and if you require a shared mailbox that is larger than 50GB, an Exchange Online Plan 2 license will need to be assigned to it. Once a license is assigned, the mailbox size should quickly increase to 100GB (equivalent size for an Exchange Online Plan 2 licensed user mailbox).

To help you with understanding how this change might impact your existing shared mailboxes and what the behavior is going forward, we have prepared the following Q&A:

When the 50GB shared mailbox creation correction goes into effect, will all the already created unlicensed shared mailboxes with a default size of 100GB be automatically switched to 50GB?

No, not automatically; but they could revert to 50GB under certain situations, read on to understand these scenarios.

When would a currently unlicensed shared mailbox lose its existing 100GB status?

Changing the state of the license on the mailbox will apply the corrected 50GB default state. If license on the mailbox is changed, the 100GB status for that mailbox will be irreversibly lost and to increase the mailbox size back to 100GB, an appropriate license will be required.

I currently have an unlicensed 100GB shared mailbox with 80GB of data and I need archiving for it. What will happen if I assign the Exchange Online Archiving license?

Any change to the licensing state of the shared mailbox will make the shared mailbox lose its 100GB status. In other words – even assigning the Archiving license only to a currently unlicensed shared mailbox will change the mailbox quota to 50GB and an Exchange Online Plan 2 license (or equivalent) will be needed to increase the mailbox size to 100GB. In this scenario, if you wanted to assign an Archiving license, you would also need to assign at least an Exchange Plan 2 license.

When this change goes into effect, let’s say I have an 80GB user mailbox and want to convert it to a shared mailbox. What will happen?

When mailbox conversion happens, the currently assigned license will stay assigned on the mailbox (by default) and shared mailbox will keep sending/receiving email. If you then remove the license from this converted shared mailbox, the 50GB mailbox quota will get enforced. The mailbox size will still be 80GB, but because the mailbox will be over quota, sending and receiving of email will not work (incoming email will error with “Mailbox is full” error, code 5.2.2). Assigning the required license to the mailbox again will resolve this problem (this change might take a little bit of time). If you intend to use a shared mailbox larger than 50GB actively, we recommend that you keep the license assigned after conversion so there are no work disruptions. You are also able to leverage Inactive Mailboxes if you just need to keep mailbox data around in inactive state.

If I have a user mailbox that is under 50GB in size and convert it to a shared mailbox – what will happen?

When mailbox conversion happens, the license will stay assigned on the mailbox (by default). If you remove the license, the mailbox will still be available for sending and receiving email because the size of the converted shared mailbox is under 50GB and shared mailboxes under 50GB do not require a license. If the size of this shared mailbox goes over 50GB, you will need to add a license to it (to increase its size to 100GB) or remove some email from it (to stay under the 50GB).

If I have a shared mailbox that is approaching 50GB and need it to be larger, what should I do?

Simply assign it an Exchange Online Plan 2 license; you do not need to do anything else. Shared mailbox size will be increased to 100GB automatically within 15 minutes.

Licensing change already went into effect. I just converted a 50GB unlicensed shared mailbox into a user mailbox and now I have a 50GB user mailbox. What’s going on?

When you convert an unlicensed mailbox from shared to user (using Exchange Admin Center, for example), the resultant mailbox will be an unlicensed user mailbox (because the starting shared mailbox did not have a license). This mailbox is not ‘attached’ to an account as there is no license assigned. When the license is assigned to the user, the mailbox will be re-attached to the account and its size will be set according to the license that was assigned to it. So, if you assign a license that gives the user a 100GB mailbox, the mailbox size will be increased to 100GB. Reminder: without a license, such an ‘unattached’ mailbox will be deleted after 30 days (as it does today).

I have an 80GB shared mailbox on-premises and I need to migrate it to Exchange Online. What should I do?

Because the target Exchange Online shared mailbox needs to be larger than the default (unlicensed) 50GB size, the shared mailbox cloud MailUser object needs to be licensed before migration is attempted. Otherwise, the migration attempt will immediately fail with the “Mailbox size exceeds target quota” error. Once the Exchange Online Plan 2 license is assigned, migration can be started.

How can I find out what is the true size of my shared mailboxes?

The easiest way to accomplish this might be to download this script. There are two things to keep in mind:

  • This will not show you sizes of mailboxes that are currently soft deleted
  • If you have shared mailboxes that were not yet logged into, you might get an error during script execution for that particular mailbox; this is just cosmetic

Nino Bilic

Comments (3)

  1. Satyajit321 says:

    And strangely there is no usage reporting available for shared mailboxes.

    Using Get-MailboxStatistics with the sessions expiring and I\O throttling isn’t the best of experiences.

    Graph API getMailboxUsageDetail isn’t very promising as well.

    If this has been already analysed for each tenant, how about sharing the data as well.

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