How to fix Outlook 'Mailbox Full' warnings and manage storage limits

Summary

This guide helps you troubleshoot and resolve: How to fix Outlook 'Mailbox Full' warnings and manage storage limits. Follow the steps below to fix the issue.

Common Causes

Receiving a 'Mailbox Full' or 'Storage Quota Exceeded' warning in Outlook means you have reached the limit of your Microsoft 365 license. When this happens, you will be unable to send or receive new emails until space is cleared. This guide will help you identify what is consuming your storage and provide practical steps to reduce your mailbox size.

Quick Fix Steps

If you need to free up space immediately to resume email flow, try these three rapid actions:

• Empty the Deleted Items folder: Right-click Deleted Items and select Empty Folder.
• Clear the Junk Email folder: Right-click Junk Email and select Empty Folder.
• Sort by size: In your Inbox, click the Filter or Sort by dropdown and select Size to quickly identify and delete the largest emails.

Detailed Instructions for Managing Storage

Analysing your storage usage

Before deleting files, it is important to understand whether your primary mailbox or your OneDrive (which now shares storage for attachments in some M365 plans) is full.

1. Log in to your account via outlook.office.com.
2. Click the Settings (gear icon) in the top right corner.
3. Navigate to General > Storage.
4. Review the breakdown of space used by individual folders and the total quota available.

Cleaning up large attachments

Attachments are typically the primary cause of storage issues. Rather than deleting the entire email, you can remove the attachment while keeping the message text.

1. Open the email containing the large attachment.
2. Click the drop-down arrow next to the attachment name.
3. Select Remove Attachment.
4. Save the email to commit the change.

Using the Mailbox Cleanup Tool

Outlook provides a built-in tool to find and manage old or large items across your entire account.

1. In the Outlook desktop app, go to the File tab.
2. Select Tools > Mailbox Cleanup....
3. Use the Find items older than option to locate outdated correspondence.
4. Use Find items larger than to target high-capacity files.

Archiving emails to a local data file

If you cannot delete emails for compliance or business reasons, you can move them from the server to a local .pst file on your computer. This removes them from the cloud storage limit while keeping them accessible in your Outlook profile.

1. Go to File > Info > Tools > Clean up Old Items (or Archive).
2. Select the folders you wish to archive.
3. Choose a date to archive items older than.
4. Specify the location of the .pst file on your local drive.

Troubleshooting Common Issues

I deleted items but the warning won't go away

Storage quotas do not always update in real-time. It may take up to 24 hours for the Microsoft 365 server to register the reclaimed space and remove the warning banner. If you have emptied your Deleted Items and Junk Email folders, you have done the necessary steps.

Managing folders with Rules

If you use Organizing Emails with Rules and Alerts in Outlook, check your automated folders. Often, rules move emails into subfolders that are forgotten and accumulate thousands of unnecessary messages over time.

Send/Receive Errors

If your mailbox is completely full, you may experience Troubleshooting Outlook Send/Receive Errors because the server will reject any incoming data. Clearing space is the first step to resolving these errors.

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