How to manage the size of a mailbox in Microsoft 365

Microsoft 365 plans have some limits related to the size of mailboxes, these limits depend on two variables: the subscription plan assigned to the account and the type of mailbox. Learn in this article how to check the maximum mailbox size, how to increase the size and how to check the current use of the mailboxes and to prevent them from exceeding the available limit.

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Maximum mailbox size in Microsoft 365

The maximum mailbox size for most mailboxes is 50 GB. There are a few exceptions, however:

  • The size limit for user mailboxes can be increased to up to 100 GB if a Microsoft 365 E3 or E5 (Exchange Online Plan 2) license is assigned. An F3 license (kiosk) includes only 2 GB of storage and does not provide archival mailboxes.
  • Public folder mailboxes (on all subscription plans) are limited to 100 GB.
  • Shared mailboxes do not require any licenses and have a standard size limit of 50 GB. However, when assigned to an E3 or E5 license, they can store up to 100 GB of data. The storage space of the user and shared mailboxes can be extended by an In-Place Archive mailbox. After the archiving feature is enabled, an archiving mailbox is assigned to each mailbox separately. By default, an archive mailbox has a 50 GB storage limit, but there is an option of unlimited archiving space thanks to an Exchange Online Plan 2 license or an Exchange Online Plan 1 license with a license Exchange Online Archiving companion. To enable archiving for a shared mailbox, the same type of license must be assigned, such as the mailbox.

Increase mailbox size in Microsoft 365

There are two ways to increase the maximum mailbox size for a user:

  • Assign an Exchange Online Plan 2 license to your mailbox. This will increase the mailbox storage limit from 50 GB to 100 GB.
  • Enable Exchange Online archiving. Depending on your Microsoft 365 license, this will give you an additional 50 GB of space or virtually unlimited space with unlimited archiving or auto-expandability.

Archiving in Microsoft 365

Archive mailboxes are attached to other mailboxes. The data stored in an archive mailbox can be accessed and searched by the user. In order for users to access the In-Place Archive from the shared mailbox, they must be given explicit FullAccess permissions to the shared mailbox. All archived data can also be found with eDiscovery. It is important to know that the mailbox items in the primary mailbox do not automatically flow into the archives.

There are two ways to move these items to the archive mailbox - either by setting up an archive policy or by moving items manually. An archiving policy is a type of retention policy. Retention policies can be defined according to your organization's needs to control your data. Mailbox items are assigned specific retention tags, according to which older items are automatically moved to archived mailboxes or deleted.

What is unlimited archiving?

Unlimited archiving in Microsoft 365 is a feature available for mailboxes using Exchange Online Plan 2 or Exchange Online Plan 1 with a complementary Exchange Online Archiving license. It needs to be enabled, just like standard archive mailboxes. Although this option provides users with the automatic expansion archiving feature, it does not mean that they should stop cleaning their mailboxes. Unlimited storage looks like fun until the user needs to find a specific email.

How to check the mailbox size in Microsoft 365

Now, of course, you can use the Exchange admin center to check a person's mailbox usage:

But if you want to check the mailbox size for all mailboxes (including shared mailboxes and resource mailboxes), using PowerShell is much easier:

First, connect to Exchange Online via PowerShell .

Then, you can download the data from the current mailbox into a variable, using the following cmdlet:

$exomailboxsize = (Get-EXOMailbox | Get-EXOMailboxStatistics)

Then, for example, you can sort mailboxes according to their size:

$exomailboxsize | select DisplayName, TotalItemSize | sort -Property TotalItemSize -Descending

And if there is a mailbox close to the maximum size, you can take steps to increase your limits or free up space.

Enable archiving mailboxes

You can enable archiving mailboxes using the Exchange admin center:

Or via PowerShell.

To enable file mailbox for a specific user or shared mailbox, use the following cmdlet:

Enable-Mailbox -Identity mailbox -Archive

You can easily check which mailboxes already have archiving enabled using:

Get-Mailbox -Archive

And to activate unlimited archiving, use the following command:

Enable-Mailbox -Identity user -AutoExpandingArchive

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Felipe Santos
Felipe Santos is a Cloud and Security Architect, with experience in Windows Server, Cluster, Storages, Backups Veeam and Office 365 environments.
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