How to fix Outlook showing 'Your mailbox is full' warnings and manage mailbox storage limits in Microsoft 365

Summary

This guide helps you troubleshoot and resolve: How to fix Outlook showing 'Your mailbox is full' warnings and manage mailbox storage limits in Microsoft 365. Follow the steps below to fix the issue.

Common Causes

If Outlook keeps telling you that your mailbox is full, you will not be able to send or receive new email until you free up some space or your Microsoft 365 administrator adjusts your storage quota. This guide walks you through clearing out large messages, setting up automatic archiving, and checking your mailbox size so you can get back to work quickly.

Quick Fix Steps

If you just need to clear the warning fast, work through these steps in order:

1. Check your current mailbox size and storage limit.
2. Sort your mailbox by size and delete or download the largest messages.
3. Empty your Deleted Items and Junk Email folders.
4. Set Outlook to download only recent emails (for Outlook desktop only).
5. Enable Online Archive or AutoArchive to move older mail automatically.

Check Your Mailbox Size and Storage Limit

Before you start deleting things, it helps to know how much space you have used and how much you have available.

In Outlook on the Web (OWA)

1. Sign in at outlook.office.com.
2. Click the gear icon in the top right, then select General > Storage.
3. Review the bar showing your used storage versus your total quota.
4. Click Manage storage to see a breakdown by folder and by large messages.

In New Outlook or Classic Outlook for Windows

1. Right-click your account name in the folder pane and choose Data File Properties (Classic) or Account Properties (New Outlook).
2. Select the Mailbox tab to view Server Quota and current usage.

Find and Remove Large Messages

The fastest way to reclaim space is to find the biggest messages and deal with them first.

Using Outlook on the Web

1. Click the gear icon > General > Storage.
2. Click Manage storage, then select Mailbox in the left column.
3. Switch the sort order to Size so the largest items appear at the top.
4. Tick the box next to each message you want to remove, then click Delete at the top.
5. Open the Deleted Items folder and click Empty folder.

Using Classic Outlook for Windows

1. Click in the search box at the top of the folder list.
2. Type largerthan:5M (change the size as needed) and press Enter.
3. Sort the results by clicking the Size column heading.
4. Select the messages you no longer need, press Delete, then empty the Deleted Items folder.

Using Outlook for Mac

1. Click in the Search field at the top of the message list.
2. Enter size:>5MB and press Return.
3. Select the largest messages, right-click and choose Delete.
4. Right-click the Deleted Items folder and choose Empty Deleted Items.

Empty Deleted Items and Junk Email

Deleted messages still take up space until you empty the folder.

1. In Outlook on the Web, right-click Deleted Items and choose Empty folder.
2. Repeat the same step for the Junk Email folder.
3. In Classic Outlook, press Ctrl + Shift + Del to open the Empty Folder dialog, then choose Empty the Deleted Items folder.

Reduce Local Cache Size in Classic Outlook

Classic Outlook downloads a copy of your mailbox to your computer. You can shrink this cache to reduce local storage use.

1. Click File > Account Settings > Account Settings.
2. Select your Microsoft 365 account and click Change.
3. Drag the Mail to keep offline slider to a smaller value, such as 6 or 12 months.
4. Click Next, then Finish and restart Outlook.

Set Up AutoArchive in Classic Outlook

AutoArchive moves older items to a local archive file so your main mailbox stays lean.

1. Click File > Options > Advanced.
2. Under AutoArchive, click AutoArchive Settings.
3. Tick Run AutoArchive every and choose a frequency (14 days is a common choice).
4. Tick Prompt before archiving so you can review what will be moved.
5. Click OK to save.

Enable the Online Archive Mailbox

Microsoft 365 includes an Online Archive mailbox that gives you extra cloud storage. Your IT administrator must enable it first.

1. In Outlook on the Web, right-click Archive in the folder pane (if visible) and choose Manage archive.
2. Set a retention policy, for example, move items older than 12 months to the archive.
3. In Classic Outlook, the Online Archive appears as a separate mailbox once enabled.

Manage Mailbox Storage in the Microsoft 365 Admin Centre

Administrators can review and adjust mailbox quotas from the admin portal.

1. Go to admin.microsoft.com and sign in.
2. Expand Users and click Active users.
3. Select the affected user, then open the Mail tab.
4. Click Manage mailbox storage to view current usage or change the quota.

Troubleshooting

The warning will not go away after deleting messages

• Empty the Deleted Items and Junk Email folders, then wait a few minutes for the server to update.
• Sign out of Outlook on all devices, wait 30 seconds, then sign back in to force a sync.
• Check the Recoverable Items folder in Outlook on the Web (under Deleted Items) and purge anything you no longer need.

I cannot find any large messages to delete

• Check the Sent Items folder for old messages with large attachments.
• Look in custom folders you may have forgotten about, including subfolders of your inbox.
• Review the Notes and Calendar items, which can also count towards mailbox usage.
• Use the Mailbox tab in Outlook on the Web to see a breakdown of storage by folder.

My administrator needs to increase my quota

• Open a support ticket and include your current usage and the reason you need additional storage.
• Your administrator can review whether an Online Archive licence or a quota increase is appropriate.

Still Having Issues?