You receive a warning that your Outlook mailbox is full and cannot send or receive new messages. This happens when your mailbox size exceeds the storage quota set by your email administrator or service provider. This article provides the steps to identify what is using space and permanently remove items to get back under your limit.
Key Takeaways: Freeing Up Outlook Mailbox Space
- File > Info > Mailbox Cleanup > View Mailbox Size: Shows a detailed breakdown of which folders are consuming the most space in your mailbox.
- Search Folder > Large Mail: Creates a dynamic folder that automatically lists all emails larger than a size you specify for quick deletion.
- File > Options > Advanced > AutoArchive Settings: Configures Outlook to automatically move old items to a local archive file to keep your main mailbox small.
Understanding Your Outlook Mailbox Quota
Most business and organizational email accounts have a storage limit. This quota is set on the mail server, not within Outlook itself. When you reach this limit, the server blocks new incoming emails and often prevents you from sending messages. The warning you see in Outlook is a notification from the server.
The primary consumers of space are email messages with large attachments, your Sent Items folder, and the Deleted Items folder. Calendar items, contacts, and tasks use minimal space. A full mailbox typically results from years of accumulated data without regular cleanup or the use of an automatic archiving tool.
How Outlook Data Files Work
If you use a POP3 account, your emails are stored in a local Outlook Data File with a .pst extension. For Microsoft 365 or Exchange accounts using Cached Exchange Mode, a local copy is kept in an Offline Outlook Data File (.ost). However, the quota limit applies to the data on the server. Deleting items only from the local cache does not free up server space. You must ensure deletions are synchronized to the server.
Steps to Identify and Remove Large Items
Follow these steps in order to effectively reduce your mailbox size. Start by analyzing what is taking up space before you begin deleting.
- Check your current mailbox size
Go to File > Info. Click the Tools dropdown and select Mailbox Cleanup. In the dialog box, click View Mailbox Size. This opens a detailed view showing the size of each folder, including the Server Data tab which shows the size on the server. - Create a Search Folder for large emails
Right-click on Search Folders in the folder pane and select New Search Folder. Scroll down and select Large mail. Click Choose and set a minimum size, such as 250 KB. Click OK. This new folder will list all emails larger than your specified size across your mailbox. - Sort and delete large emails
Open the new Large Mail search folder. Sort by Size or Received date. Select emails with large attachments you no longer need. Press the Delete key. Remember to empty the Deleted Items folder afterward. - Clean the Sent Items and Deleted Items folders
These folders are often the largest. Open Sent Items, sort by Size, and delete old sent messages with attachments. Then, right-click the Deleted Items folder and select Empty Folder to permanently remove all items waiting there. - Compact your local data file for POP3 accounts
For POP3 accounts, deleting items does not immediately reclaim disk space from the .pst file. Go to File > Account Settings > Account Settings. Select the Data Files tab, choose your .pst file, and click Settings. Go to the General tab and click Compact Now to reduce the file size.
Using the Archive Feature Manually
Archiving moves old items from your main mailbox to a separate local Outlook Data File (.pst). This removes them from the server, freeing up quota space.
- Initiate manual archiving
Go to File > Info > Tools > Clean up old items. Alternatively, go to File > Info > Cleanup Tools > Archive. - Configure the archive settings
In the Archive dialog, select Archive this folder and all subfolders and choose your main mailbox. Set the date condition, such as items older than 6 months. Ensure the archive file path is correct and click OK.
If You Still Cannot Free Enough Space
Outlook Says Mailbox is Full After Deleting Items
If you delete items but the quota warning persists, the Deleted Items folder may not have been emptied. Items moved there still count against your quota. You must right-click the Deleted Items folder and select Empty Folder. Also, allow time for the server to synchronize the changes, which can take several minutes.
Large Shared Mailboxes or Public Folders
If you have full access to a shared mailbox, its size does not count against your personal quota. However, if you are using the shared mailbox in a way that downloads its contents into your local profile, it can appear to consume space. Focus your cleanup efforts on your primary mailbox listed under your name.
Archiving Does Not Reduce Server Mailbox Size
This happens if AutoArchive was configured to move items to another folder within your main mailbox instead of to a separate .pst file. Verify the archive location by going to File > Options > Advanced > AutoArchive Settings. The Move old items to field should point to a file path, not a mailbox folder.
Manual Cleanup vs. AutoArchive: Key Differences
| Item | Manual Cleanup | AutoArchive |
|---|---|---|
| Process | User-initiated, one-time action | Automatic, scheduled process |
| Control | Full user selection of items and folders | Rules-based on item age and folder policy |
| Best for | Immediate quota crisis, large attachments | Ongoing maintenance, old emails |
| Storage location | Items are permanently deleted | Items moved to a local archive .pst file |
| Server space | Freed immediately after emptying Deleted Items | Freed when archive runs and syncs |
You can now clear your mailbox quota by using the Mailbox Cleanup tool and Search Folders. Set up AutoArchive to prevent the problem from recurring. For advanced management, use the Outlook web app to sort your entire mailbox by size and filter for specific attachment types.