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How to Resolve the “Your Mailbox is Almost Full” warning in Outlook 365

Are you tired of constantly receiving those “Your mailbox is almost full” emails from the Outlook team? It can be quite frustrating, not to mention alarming. The message is crystal clear – your Outlook storage space is running dangerously low.

Outlook 365 houses all your essential mailbox data – emails, contacts, calendars, tasks, journals, and notes, among other things – in a file called OST, which has a capacity limit of 50 GB. However, problems start creeping in when your OST file swells beyond 10 GB. You might start noticing issues like send/receive errors, sluggish search results, and painfully slow Outlook loading times.

When your OST file becomes excessively large and surpasses the limit set by your administrator (which can be 10 GB, 20 GB, or even more), that’s when you’ll start receiving the dreaded “your mailbox is almost full” warning message. This is a red flag you shouldn’t ignore because it can lead to disruptions in your email services, causing Outlook to be unable to send or receive new messages.

How to Resolve the "Your Mailbox is Almost Full" warning in Outlook 365

The good news is, there are several effective ways to tackle the problem of OST file storage space. These solutions include deleting unnecessary messages, clearing out junk folders, and even disabling Cached Exchange Mode, among others. In this blog, we’ll delve into these methods in detail to help you reclaim your Outlook 365 storage space and ensure seamless email operations. So, let’s dive right in and address that nagging “Your mailbox is almost full” issue once and for all.

Understanding the Triggers for Outlook 365 Mailbox Full Errors

In the bustling world of Outlook 365, where hundreds of emails are zipping back and forth daily, your storage space can gradually dwindle. It’s not just emails, but hefty attachment files can also push your mailbox towards the brink of capacity. So, it’s crucial to maintain an organized mailbox to ensure you have ample storage space for your Outlook 365 profile.

Let’s explore the key reasons behind the infamous “Outlook 365 mailbox is almost full” error:

  • Large OST Files: The size of your Outlook OST (Offline Storage Table) file can be a major culprit behind performance issues in Outlook 365. Typically, each Outlook profile is granted a storage limit of up to 50 GB, and in some exceptional cases, even more. When this storage capacity is exhausted, you’ll start encountering the “mailbox full” error message.
  • Deleted Emails: Often, users receive emails that are irrelevant or no longer needed. In an attempt to declutter their inbox, they hit the delete button without realizing that these discarded emails find their way into the deleted items folder. Over time, these seemingly innocuous deleted emails can accumulate and consume a significant chunk of your storage space, eventually leading to the dreaded “Outlook 365 mailbox full” error.

Understanding the Triggers for Outlook 365 Mailbox Full Errors

Now, let’s delve into these triggers more deeply and explore how you can effectively manage them to prevent mailbox full errors and ensure smooth sailing with your Outlook 365 experience.

Effective Solutions to Resolve the “Your Mailbox is Almost Full” Error in Outlook 365

Is your Outlook 365 mailbox feeling a bit cramped? Don’t worry; there are several methods to free up space and bid farewell to that pesky “Your Mailbox is Almost Full” error. In this post, we’ll guide you through the steps to clean up, compact, and reduce your mailbox size in Outlook 365, ensuring a smoother email experience.

Note: Be cautious of suspicious messages claiming your mailbox is full. Spammers and hackers can send such messages. Always verify if the email is from the official Office/Outlook team. Avoid clicking on any links within these emails, as they may pose security risks to your account.

Solution 1: Streamline Your Outlook Inbox with the Mailbox Cleanup Tool

Is your Outlook inbox becoming a cluttered storage space? Fear not, as Outlook offers a handy Mailbox Cleanup tool that can help you regain control and create some much-needed breathing room. Let’s walk through the steps to use this tool effectively:

Step 1: Open Outlook and navigate to the “File” menu.

Step 2: From the “Info” tab, locate the “Tools” drop-down menu.

Step 3: In the menu that appears, select the “Mailbox Cleanup” tool.

In the menu that appears, select the "Mailbox Cleanup" tool.

Once you’ve entered the Mailbox Cleanup tool, you can take several actions to declutter your inbox and free up valuable storage space:

  • Clear Your Deleted Items Folder: Your Deleted Items folder can harbor a surprising amount of clutter, including emails and conversations you thought were gone for good. To clear it out, simply click on the “Empty” button next to the “Emptying the deleted items folder permanently deletes those items” option. If you want to gauge the size of your deleted items first, you can click on the “View Deleted Items Size” button.
  • Auto-Archive Old Emails: To keep your primary inbox tidy and organized, you can set up an Auto-Archive feature for your older emails. Click on the “AutoArchive” button within the Mailbox Cleanup dialog window to get started.
  • Identify and Remove Large Emails: Another effective way to declutter your Outlook inbox is to pinpoint and eliminate large emails that may be hogging space unnecessarily. Using the Mailbox Cleanup tool, you can search for emails exceeding a specific size limit and delete those you no longer need. Additionally, you can identify emails older than a certain number of days and remove them as well.

To find and delete these space-consuming items:

  • Enter the desired number of days in the “Find items older than” box and click on the “Find” button.
  • Alternatively, enter a specific size in the “Find items larger than” box and hit “Find.” The tool will then locate these items for you.
  • Select the items you wish to remove and proceed with their deletion.

Streamline Your Outlook Inbox with the Mailbox Cleanup Tool

By following these steps, you can efficiently manage your Outlook inbox, create more room for essential emails, and ensure that your mailbox operates smoothly. Let’s get started on your inbox cleanup journey!

Solution 2: Tackle Large Emails in Outlook without the Mailbox Cleanup Tool

If you want to take a more direct approach to address the issue of large emails in Outlook, you can follow these steps to identify and delete them:

Step 1: Launch Outlook and navigate to the “Home” tab.

Step 2: From the left-side pane, click on “Search Folders.” Next, select the “New Search Folder” option.

From the left-side pane, click on "Search Folders." Next, select the "New Search Folder" option.

Step 3: Under the “Organizing Mail” section, choose the “Large mail” option.

Under the "Organizing Mail" section, choose the "Large mail" option.

Step 4: Beneath the “Customize Search Folder” option, click the “Choose” button.

Step 5: In the “Show mail greater than” box, specify the size limit for the emails you want to identify. Then, click the “OK” button.

Step 6: The search will display all the emails in your mailbox that exceed the specified size limit. You can review the list and decide which emails you no longer need. Then, proceed to delete them.

By following these straightforward steps, you can efficiently identify and eliminate large emails from your Outlook mailbox, helping you reclaim valuable storage space and reduce the risk of encountering mailbox full errors. Let’s get started on decluttering your mailbox!

Solution 3: Eliminate Duplicate Emails in Outlook

Duplicate emails can be a silent space-hogging culprit in your Outlook inbox. To efficiently tackle this issue, follow these steps:

Step 1: In Outlook, navigate to the “Home” tab. Ensure that you have selected the Inbox folder where you want to remove duplicates.

Step 2: Click on the “Delete” drop-down button located in the toolbar.

Step 3: From the drop-down menu, select the “Clean Up Folder” option.

Eliminate Duplicate Emails in Outlook

Step 4: A confirmation prompt will appear. To proceed with the removal of redundant and duplicate conversations, click the “Clean Up Folder” button in the prompt.

Outlook will now diligently clear out all the unnecessary duplicate emails and conversations, helping you declutter your mailbox and free up valuable storage space. Enjoy a cleaner and more organized inbox!

Solution 4: Streamline Deleted Items Management in Outlook

To effortlessly and automatically clear your Deleted Items folder in Outlook every time you close the application, you can configure your settings. This simple adjustment ensures that items in the trash are permanently deleted, helping you optimize storage space in Outlook. Here are the steps to set this up:

Step 1: Open Outlook and click on the “File” menu.

Step 2: From the menu, select “Options” and navigate to the “Advanced” tab in the Outlook Options window.

Step 3: Under the “Outlook start and exit” section, locate and check the “Empty Deleted Items folders when exiting Outlook” box.

Streamline Deleted Items Management in Outlook

Step 4: Finally, confirm your settings by pressing the “OK” button.

With these settings in place, Outlook will automatically clear your Deleted Items folder every time you close the application, ensuring that no unnecessary items linger and optimizing your storage space. Enjoy a tidier Outlook experience!

Solution 5: Streamline Your Outlook 365 Storage with Email Archiving

To efficiently manage your Outlook 365 storage space, consider archiving older emails. This process can help you keep your primary inbox clutter-free while preserving important messages for future reference. Follow these steps to archive mailbox items in Outlook 365:

Step 1: Open Outlook and navigate to the “File” tab.

Step 2: Select “Info,” then click on “Cleanup Tools,” and choose “Archive.”

Step 3: In the Archive dialog box, verify your Archive settings. Ensure that the “Archive this folder and all subfolders” option is selected.

Streamline Your Outlook 365 Storage with Email Archiving

Step 4: Choose the specific mailbox items you wish to archive from your Outlook profile. You can opt to archive emails, contacts, calendars, and more.

Step 5: Select the date from which you want to start archiving older items.

Step 6: Click the “Browse” button to choose a location where your archived items will be saved.

Step 7: To prevent automatic archiving, make sure to mark the “Do not auto-archive” checkbox.

Step 8: Finally, click the “OK” button to initiate the archiving process.

By following these steps, you can effectively manage your Outlook 365 storage space by archiving older items, keeping your inbox organized, and ensuring that you have ample room for new emails. Enjoy a more streamlined and efficient Outlook experience!

Solution 6: Streamlining Storage by Removing Oversized Emails

Large emails and attachments can quickly eat up your Outlook 365 mailbox storage space. To regain control and free up valuable storage, you can follow these steps to identify and remove oversized items:

Step 1: Launch Outlook 365 and locate “Search Folders” in the folder pane.

Step 2: Click on “New Search Folder.”

Step 3: Under the “Organizing Mail” section, choose the “Large Mail” option.

Step 4: Select “mail at least this large” and specify the size threshold for the emails you want to identify as oversized.

Step 5: Press “OK” to complete the setup.

Outlook 365 will create a new folder under your profile automatically, containing emails larger than the specified size threshold. You can then review and remove these oversized emails, effectively optimizing your mailbox storage space.

By following these steps, you can efficiently manage your Outlook 365 storage, ensuring that large emails and attachments no longer burden your mailbox. Enjoy a more streamlined and spacious Outlook experience!

Solution 7: Optimize Storage by Moving Attachments to OneDrive

Saving email attachments to your OneDrive account is an efficient way to free up storage space in Outlook 365. Follow these steps to transfer attachments to OneDrive:

Step 1: Start by clicking on the “Filter” option located at the top of your email list.

Step 2: Next, select “Sort by” and choose “Size.”

Note: If your emails are sorted in ascending order, click on “Filter” > “Sort by” > “Size” again.

Step 3: Identify the email that contains the attachment you want to move.

Step 4: Click on “Save to OneDrive” located under the attachment.

Step 5: After saving the attachment to OneDrive, you can safely delete the original email.

Step 6: Repeat this process for any other emails containing attachments that you want to move to OneDrive.

By following these steps, you can efficiently transfer attachments to your OneDrive account, reducing the burden on your Outlook 365 storage and ensuring a more organized and spacious mailbox. Enjoy the benefits of a streamlined email experience!

Solution 8: Enhance Performance and Save Space by Disabling Cached Exchange Mode

If you’ve enabled Cached Exchange Mode in your Outlook 365 profile, it’s essential to be aware that this can consume a significant amount of storage space on your computer. Additionally, when working with a shared mailbox and Cached Exchange Mode is active, it can lead to a storage crunch and negatively impact your email client’s performance. To address this issue, follow these steps to disable Cached Exchange Mode:

Step 1: Start by clicking on the “File” tab in Outlook.

Step 2: Select the “Account Settings” profile icon.

Step 3: Locate and click on the Microsoft or Exchange account that you want to modify, then click on the “Change” option.

Enhance Performance and Save Space by Disabling Cached Exchange Mode

Step 4: Under “Exchange Account Settings,” uncheck the “Use Cached Exchange Mode to download email to an Outlook file” option.

Under "Exchange Account Settings," uncheck the "Use Cached Exchange Mode to download email to an Outlook file" option.

Step 5: Confirm your changes, then restart Outlook.

By following these steps, you can optimize the performance of Outlook 365 and alleviate the storage burden created by Cached Exchange Mode. This will allow you to continue sending and receiving emails smoothly. As an additional long-term solution for the “Outlook 365 mailbox full” error, users can consider recreating OST files.

Solution 9: Optimize Storage by Compressing the Outlook PST File

Compressing the Outlook PST (Personal Storage Table) file is a practical step to alleviate storage issues and potentially resolve the “Outlook 365 mailbox full” error. Here are the steps to compress your PST file:

Step 1: Start by opening Outlook and clicking on the “File” menu.

Step 2: Navigate to the “Account Settings” option and select “Account Settings” again.

Step 3: In the “Data Files” tab, choose the Outlook data file (PST) that you want to compress.

Step 4: Click the “Settings” button.

Optimize Storage by Compressing the Outlook PST File

Step 5: Within the PST file settings, click the “Compact Now” button.

Step 6: Wait for the compression process to complete.

Step 7: Once the PST file is successfully compressed, check to see if the “Outlook 365 mailbox full” issue is resolved.

Compressing the PST file can help optimize storage space and improve Outlook’s performance. By following these steps, you can potentially resolve the storage-related problems and enjoy a more efficient Outlook experience.

Solution 10: Adjusting PST File Size Limit in Registry Editor

If you’re still encountering storage issues in Outlook 365, you can try increasing the size limit for PST files through the Registry Editor. Please note that making changes in the Windows Registry can affect your system, so proceed with caution. Here’s how you can attempt this solution:

Step 1: Press the Windows key + R to open the Run dialog box.

Step 2: Type “regedit” and press Enter to open the Registry Editor.

Step 3: Navigate to the following registry key: HKEY_CURRENT_USER\Software\Microsoft\Office\<Your Outlook Version>\Outlook\PST

Note: Replace “<Your Outlook Version>” with the version you’re using (e.g., 2019, 2016, etc.).

Step 4: In the right pane, locate and double-click the “MaxLargeFileSize” entry.

Step 5: Change the value to increase the PST file size limit. The default value is usually set in MB, so you can raise it as needed. For example, setting it to 30,000 will allow a 30 GB PST file.

Step 6: Similarly, find and double-click the “WarnLargeFileSize” entry, and adjust the value to your preferred size.

Step 7: Close the Registry Editor.

Step 8: Restart Outlook to apply the changes.

By adjusting these values in the Registry Editor, you can increase the PST file size limit in Outlook, potentially resolving storage issues. However, be cautious when modifying the Windows Registry, as incorrect changes can harm your system.

Solution 11: Expanding Storage with an Upgrade

If you find that your account is consistently running low on storage space, you have the option to upgrade to a paid plan. Upgrading your account can provide a more extensive storage solution and additional benefits. Here’s how you can do it:

Step 1: Visit the website and sign in to your account.

Step 2: Look for the option to upgrade or expand your account storage. This might be labeled as “Upgrade” or “Premium.”

Step 3: Follow the on-screen instructions to choose a suitable plan and make the necessary payment.

Upgrading your account can provide you with a more substantial mailbox capacity, enhanced email security, and an ad-free inbox. This can be an effective solution if you consistently run out of space in your email account. Enjoy the increased storage and additional benefits!


In the world of Outlook 365, where a maximum of 50 GB is granted for storage, effective organization of your mailbox becomes paramount. In this comprehensive guide, we’ve explored various strategies to help you optimize your Outlook 365 storage space.

By implementing these solutions, you can ensure that your Outlook 365 mailbox remains organized, efficient, and equipped to handle your email needs. Enjoy a seamless Outlook experience with ample storage space at your disposal!

Frequently Asked Questions (FAQs)

Question: Is it possible to increase the mailbox size in Office 365 to more than 100GB?

Answer: Unfortunately, you cannot exceed a mailbox size of 100GB in Office 365. This 100GB limit applies to all Office subscription plans for the primary mailbox folder. Additionally, archive mailboxes have their own storage limit, which is 50GB.

However, it’s important to note that some high-end subscription plans, such as Microsoft 365 Enterprise E3/E5 and Office 365 Enterprise E1, offer the possibility of larger storage limits, up to a whopping 1.5 terabytes (TB) for archives.

So, while most Office 365 users are restricted to a 100GB mailbox size, there are premium plans available that provide significantly more storage space, especially for archive purposes. Be sure to check the specific details of your subscription plan to understand the storage limits that apply to your account.

Alex Lim is a certified IT Technical Support Architect with over 15 years of experience in designing, implementing, and troubleshooting complex IT systems and networks. He has worked for leading IT companies, such as Microsoft, IBM, and Cisco, providing technical support and solutions to clients across various industries and sectors. Alex has a bachelor’s degree in computer science from the National University of Singapore and a master’s degree in information security from the Massachusetts Institute of Technology. He is also the author of several best-selling books on IT technical support, such as The IT Technical Support Handbook and Troubleshooting IT Systems and Networks. Alex lives in Bandar, Johore, Malaysia with his wife and two chilrdren. You can reach him at [email protected] or follow him on Website | Twitter | Facebook

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