Are you facing the "mailbox is full in Outlook" error? In this article, we will resolve the problem using the best automated and manual solutions. We will also provide preventive tips for fixing the error in the future.

If you’re a regular user of MS Outlook, then you might know that this error usually appears when your mailbox storage limit is reached, or your Outlook data file becomes too large. However, you can resolve it by removing unnecessary space and effectively managing your Outlook mailbox full error.

Let’s walk through the article by understanding what it actually means. 

What Does “Your Mailbox is Full in Outlook” Mean?

This means you can't store more data in your MS Outlook mailbox. This could be caused by various factors. Here are some given below:

• Your email provider’s storage limit (Exchange, Microsoft 365, IMAP).
• You might have multiple PST/OST data files stored on your system.
• There are too many emails with large attachments.

When this limit is reached, Outlook may stop sending or receiving emails until space is cleared.

Common Causes of Outlook Mailbox Full Error

Here are the most common reasons:

1. Large Attachments - Images, PDFs, videos, and ZIP files take up a lot of space.
2. Too Many Old Emails - Years of emails can fill storage quickly.
3. Deleted Items Folder not Removed- Deleted emails still use space.
4. Junk Folder Buildup - Spam emails also consume mailbox size.
5. Duplicate PST/OST Files - Unwanted Outlook data files grow over time.
6. Server Quota Limit - Your email plan may have a fixed storage cap.

How to Check Mailbox Size in Outlook

Before fixing the issue, check how much space your mailbox uses.

For Outlook on Windows:

1. Navigate to File > Tools > Mailbox Cleanup
2. Click on View Mailbox Size

For Outlook on Mac:

1. Click Tools > Accounts
2. Select your account > More Options > Mailbox Size

For Microsoft 365 Users:
Log in to Outlook Web > Settings > View storage usage.

This helps identify which folders consume the most space.

Quick Fixes to Free Up Mailbox Space

1. Delete Unnecessary Items

Sort emails by size and remove old or irrelevant messages. Focus on newsletters, promotions, and outdated conversations.

Pro Tip: You can use an automated solution for quick removal. We recommend using the SysTools Outlook Duplicate Remover Software.

2. Remove Large Attachments

Save important attachments to your computer or cloud storage, then delete them from emails.

3. Empty Deleted Items and Junk Folder

• Right-click on Deleted Items > Empty Folder
• Do the same for the Junk Email folder.

This instantly frees the hidden occupied space.

4. Archive Old Emails

Use Outlook’s built-in Archive feature to move older emails to a separate data file without deleting them.

Steps:

1. Go to File > Tools > Archive
2. Choose folders and date range
3. Save the archive file

5. Compact PST/OST Files

After deleting data, compact the file to reduce actual file size:

1. Nav to File > Account Settings > Data Files
2. Select PST/OST file > Settings > Compact Now

Advanced solution for Large Mailboxes

If you’re working on a large PST file that contains several duplicates, then the manual cleanup method doesn't help much, as it becomes slow. In such cases, you can use expert recommendations, such as the SysTools PST Duplicate Remover, which is an advanced automated solution.

• Eliminate duplicate items like emails, contacts, calendars, and tasks.
• Clean PST and OST files safely.
• Reduce mailbox size without deleting unique data.

This approach is especially helpful for business users and long-term Outlook users with heavy mailboxes.

Watch Video of Fixing the Mailbox is Full in Outlook Error

How to Prevent Mailbox Full Error in the Future

Follow these advanced tips to avoid the error in the future.

• Regularly clean your mailbox data.
• Turn on AutoArchive mode in MS Outlook.
• Don't import the same data file multiple times.
• Avoid receiving large or heavy attachments.
• Eliminate unwanted items to free up space
• Always use a professional solution for a faster, accurate, and more reliable result.

Conclusion

In this article, we've provided the best solution for solving the "mailbox is full" problem in Outlook. Whether you're working on a large file and are wasting time cleaning and managing it manually, we've provided the best solution for you. Instead of this, you can use the aforementioned solution suggested by professionals. We've also offered preventive tips for the future.