Managing Microsoft 365 shared mailbox storage

When a shared mailbox hits its storage limit in Microsoft 365, it stops sending and receiving email for everyone on the team. This article covers how to check your current storage, understand your limits, and keep the mailbox from filling up.

Steps labelled Admin are done by a Microsoft 365 administrator. Steps labelled User are done by the person at their own computer.

Check your current storage first

Before changing anything, see how full the mailbox is and what is filling it up.

**Admin:**Use the mailbox usage report in the Microsoft 365 admin center to see storage and activity across all your mailboxes.

**User:**You can check a shared mailbox's storage directly from Outlook on the web.

Understand your storage limits

A shared mailbox does not need its own license to exist, but the license controls how much it can hold.

Without a license assigned, a shared mailbox gets 50 GB for the main mailbox and 50 GB for the archive. Assigning an Exchange Online Plan 2 license (included in Microsoft 365 E3, E5, and similar plans) doubles the main mailbox to 100 GB and expands the archive from 100 GB up to 1.5 TB with auto-expanding archive enabled.

The archive is a second storage area attached to the mailbox. Older email moves into it automatically, freeing room in the main mailbox. Setting up the archive is the foundation for most of the tips below, so do it first.

Tip 1. Turn on an archive for the shared mailbox (Admin)

The archive gives the mailbox a separate storage area for older email. Without it, retention rules have nowhere to move email to.

1. Sign in to the Exchange admin center at admin.exchange.microsoft.com.
2. Go to Recipients, then Mailboxes.
3. Select the shared mailbox to open its details.
4. Open the Others tab.
5. Under Mailbox archive, select Manage mailbox archive.
6. Switch the toggle to On, then save.

The archive can take a few hours to appear, sometimes up to a day. A shared mailbox archive is capped at 50 GB until the mailbox has an Exchange Online Plan 2 license.

Tip 2. Turn on auto-expanding archive (Admin)

Auto-expanding archive lets the archive keep growing in 100 GB steps, up to 1.5 TB, instead of stopping at the first 100 GB. Without it, a busy mailbox can fill the archive and stall again.

Requirements: the archive must already be on (Tip 1), and the mailbox needs an Exchange Online Plan 2 license.

This setting is not available in the admin center; it must be enabled with PowerShell. An admin runs these two commands once:

Connect-ExchangeOnline

Enable-Mailbox -Identity "[email protected]" -AutoExpandingArchive

Auto-expanding archive cannot be turned off once it is on. When the archive nears full, Microsoft adds the next block of space, which can take up to 30 days to appear.

Tip 3. Set email to move to the archive automatically (Admin)

This is the rule that does the real work. It tells Microsoft 365 to move email older than a set age out of the main mailbox and into the archive, automatically, every week. This is the single most effective way to keep a shared mailbox from filling up.

Every licensed mailbox already has a built-in policy that moves email older than two years to the archive. You can use that as-is or shorten the timeframe.

1. Open the Microsoft Purview portal from the Microsoft 365 admin center.
2. Go to Data Lifecycle Management, then expand Exchange (legacy).
3. Under MRM Retention tags, review the existing tags or create a new one, for example a tag that moves all email older than one year to the archive.
4. Under MRM Retention policies, open a policy and add the tag, or create a new policy with the tags you want.
5. Apply the policy to the shared mailbox from the Exchange admin center, under the mailbox's Mailbox policies settings.

Retention rules run about once every seven days. An admin can apply them immediately with the PowerShell command Start-ManagedFolderAssistant.

Choose a move age long enough that only old, finished email moves to the archive. Active conversations your team is still working on should stay in the main mailbox. One to two years is a common, safe choice.

Tip 4. Add a license for more room (Admin)

Assigning an Exchange Online Plan 2 license to the shared mailbox doubles the main mailbox to 100 GB and unlocks the larger, growing archive.

1. In the Microsoft 365 admin center, open the shared mailbox account.
2. Go to Licenses and apps.
3. Assign an Exchange Online Plan 2 license, or a plan that includes it such as Microsoft 365 E3 or E5.
4. Save.

A license adds ongoing cost. Many teams get enough relief from the archive and retention rules alone (Tips 1 to 3) without paying for a license. Try those first.

Tip 5. Empty Deleted Items and Junk regularly (User)

Deleted and junk email still count against the mailbox limit until they are cleared out.

1. In Outlook, right-click the Deleted Items folder on the shared mailbox.
2. Select Empty Folder.
3. Do the same for the Junk Email folder.

Emptied email sits in a recovery area for about 14 days and still uses space during that window, so the room is not freed up the same day. An admin can also add a retention tag (Tip 3) to clear these folders automatically.

Tip 6. Find and clear large attachments (User)

A handful of large attachments often takes up more space than thousands of plain emails. Finding the biggest items clears the most room fastest.

1. In Outlook, click into the shared mailbox's search box.
2. Type size:>10MB and press Enter to list every message larger than 10 MB.
3. Review the results. For anything worth keeping, save the attachment to a shared SharePoint or Teams location first.
4. Delete the email, or remove the attachment from it, then empty Deleted Items (Tip 5).

Storing large files in SharePoint or Teams instead of the mailbox keeps the mailbox small and gives the whole team a shared, searchable place to find documents.

Tip 7. Do not archive to a PST file (User)

Outlook has an older feature that exports email to a PST file or auto-archives to one on a person's computer. Avoid this for a shared mailbox.

A PST file lives on one person's computer. It is not backed up, the rest of the team cannot search it, and it pulls email out of the managed mailbox. If that computer fails, the email is gone. The online archive (Tips 1 to 3) does the same job correctly: everything stays in Microsoft 365 and stays available to the whole team.

Need help? Contact [email protected].