Managing Storage in Microsoft 365 Shared Mailboxes

Shared mailboxes fill up for two separate reasons, and the fixes are different. It helps to know which problem you have before you start.

The first is mailbox storage in Microsoft 365, the space the mailbox uses on Microsoft's servers. When it fills up, the mailbox stops sending and receiving email for everyone. This is fixed through archiving, retention policies, and licensing.

The second is Outlook storage on each computer, the local copy Outlook keeps on a user's PC called the cache or OST file. When it gets too big, Outlook slows down or freezes for that one person. This is fixed in Outlook settings.

The tips below are grouped by which problem they solve. Steps labelled Admin are done by a Microsoft 365 administrator. Steps labelled User are done by the person at their own computer.

Part 1. Mailbox Storage in Microsoft 365

This is the storage that matters most. When a shared mailbox hits its limit, it can no longer receive new email and your team starts missing messages. The goal is to keep older email out of the main mailbox and give the mailbox more room to grow.

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.

Know 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, which frees room in the main mailbox. The archive is the foundation for almost everything below, so set it up 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, the retention rules in Tip 3 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 is the one setting Microsoft does not offer in the admin center; it has to be turned on with PowerShell. An admin runs these two commands once:

Connect-ExchangeOnline

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

Note that auto-expanding archive cannot be turned off once it is on. That is fine for a shared mailbox you plan to keep. 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, so changes are not instant. 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. This is the most direct way to add headroom.

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 the right way: everything stays in Microsoft 365 and stays available to the whole team.

Part 2. Outlook Storage on Each Computer

This is a separate issue from the mailbox storage above. Classic Outlook keeps a local copy of email on each person's computer, in a file called the OST or cache. When a shared mailbox is added to Outlook, by default Outlook downloads the entire shared mailbox into that local copy. With a large shared mailbox, this can make the cache huge, which slows Outlook down or causes it to freeze for that one person.

The steps in this part apply to classic Outlook for Windows. The new Outlook for Windows and Outlook on the web work directly from the server and do not keep a local cache, so these settings do not apply there.

Tip 8. Turn Off Download Shared Folders (User)

This is the main fix. It stops Outlook from copying the shared mailbox onto the local computer. Outlook still shows the shared mailbox and works normally; it just reads from the server instead of from a local copy.

1. In classic Outlook, go to File, then Account Settings, then Account Settings again.
2. On the Email tab, select your own Microsoft 365 account, then click Change.
3. Click More Settings, then open the Advanced tab.
4. Uncheck Download Shared Folders. Leave Use Cached Exchange Mode checked.
5. Click Apply, then OK, and restart Outlook.

For a step-by-step walkthrough with screenshots, see our knowledge base article on turning off cached exchange mode for shared mailboxes.

Tip 9. Keep Less Email Offline (User)

Outlook can keep less of your own email stored locally, which also keeps the cache smaller. Everything stays on the server and is still searchable.

1. Go to File, then Account Settings, then Account Settings, select your account, and click Change.
2. Find the slider labelled Download email for the past.
3. Move it to a shorter period, such as 3 months or 1 year.
4. Click Next, then Done, and restart Outlook.

Tip 10. Clear the Old Cache Once (User)

After you turn off Download Shared Folders (Tip 8), the shared mailbox content Outlook already downloaded stays in the local cache until the file is rebuilt. Rebuilding it once reclaims that space. This is a one-time cleanup. Once the setting is off, the cache will not fill up with shared mailbox email again.

1. Close Outlook completely.
2. Press Windows + R, type %localappdata%\Microsoft\Outlook, and press Enter.
3. Find the file ending in .ost and rename it, for example by adding .old to the end.
4. Reopen Outlook. It builds a fresh cache that no longer includes the shared mailbox folders.

Only the .ost file (the offline cache) is safe to rebuild this way, because it is just a copy of what is on the server. Never rename or delete a .pst file. A PST can hold the only copy of email and deleting it would lose data permanently.

Quick Reference

Start at the top. The first three tips solve most shared mailbox storage problems on their own.

Tip 1. Turn on the archive: Gives old email somewhere to go. Done by an admin in the Exchange admin center.

Tip 2. Auto-expanding archive: Lets the archive keep growing. Done by an admin via PowerShell.

Tip 3. Move email to the archive automatically: Keeps the main mailbox from filling up. Done by an admin in Microsoft Purview.

Tip 4. Add a Plan 2 license: Doubles mailbox size and unlocks the large archive. Done by an admin in the Microsoft 365 admin center.

Tip 5. Empty Deleted Items and Junk: Reclaims wasted space. Done by the user in Outlook.

Tip 6. Clear large attachments: Frees the most space fastest. Done by the user in Outlook.

Tip 7. Avoid PST archiving: Prevents lost and unmanaged email. Done by the user in Outlook.

Tip 8. Turn off Download Shared Folders: Stops Outlook slowdowns on the PC. Done by the user in classic Outlook.

Tip 9. Keep less email offline: Keeps the local cache smaller. Done by the user in classic Outlook.

Tip 10. Clear the old cache once: Reclaims space already used locally. Done by the user in Windows and Outlook.

Need a hand setting this up or have questions? Reach out to [email protected] and we will be happy to help.