How to Reduce Mailbox Size in Outlook 365 Professionally?

By default, Outlook organizes your emails by received date, with the newest emails at the top. This display method is great for productivity but offers little help when users are searching for how to reduce mailbox size in Outlook 365.
Therefore, we need to explore methods to effectively manage and reduce the size of our mailboxes. Before that let us look at the core reasoning behind this request.

Why Reduce Mailbox Size in Outlook 365?

Reducing your mailbox size might seem counterintuitive, as it can feel restrictive. However, from an organizational standpoint, there are several reasons why admins may want to impose smaller mailbox limits:

• Cost Management: High-end subscription plans cost more. Limiting mailbox sizes helps avoid unnecessary upgrades.
• Better Resource Allocation: M365 is a business suite, and using Outlook for basic email conversation and document storage isn’t efficient. Reducing mailbox limits encourages users to utilize other applications, such as OneDrive/SharePoint, for document management.
• Compliance: Strict industry guidelines or compliance protocols can require voluntary reductions in mailbox space.
• System Performance: Large mailboxes can lead to performance issues, file corruption, and slow search speeds.
• Remedy Other Errors: A mismanaged storage unit may even prevent you from fixing other errors like O365 shared mailbox folders not showing up in Outlook.

Identify Outlook Email Categories by Size to Reduce Them

When sorting by size, your emails will generally be divided into four categories:

• Small: Emails less than 25 KB.
• Medium: Emails between 25 KB and 1 MB.
• Large: Emails between 1 MB and 5 MB.
• Very Large: Emails larger than 5 MB.

Procedure for Sorting Emails by Size
The first step in managing your mailbox size is to sort your emails by size, with the largest emails at the top. Outlook allows this without the need for external tools.

1. Click on the Sort icon (Two vertical parallel arrows pointing in opposite directions).
2. Under the Sort by category, select Size.
3. The sort order will automatically place the largest items on top. You can verify this by toggling the Sort icon. again.

Note: You may not see all four categories if you don’t have emails that meet those specific size thresholds.
We recommend starting with the Very Large category and working your way down. This will allow you to achieve the maximum reduction in mailbox size in the shortest amount of time.

How to Reduce Mailbox Size in Outlook 365 by Deleting Emails

The steps for removing emails are the same regardless of the category.

1. Hover your cursor over the email you want to delete.
2. A trash bin icon will appear on the right.
3. Move your cursor onto the bin; once it changes to red and a lid-opening animation appears, click it.
4. You will receive an alert with an undo button at the bottom of your screen. You can close it early or wait for it to disappear on its own.

Speed up Deletion by using Bulk Selection of Emails
Unfortunately, there isn’t a bulk selection button at the size category level. However, you can use the following methods to select multiple emails for deletion:

• Sequential Selection: Click on one item in the list, then scroll to the bottom of the category. Hold the Shift key and click on the last item. This will select all items between and including the items you clicked.
• Staggered Selection: Hold the Ctrl key and select the relevant emails individually with your cursor.

Using Search Folders to Filter Large Emails

Another way to quickly locate large emails is by creating a search folder:

1. In the Outlook Classic client, look for Search Folders in the left menu pane.
2. Right-click on it and choose the New Search Folder… option.
3. In the New Search Folder box, scroll down to the Organizing Mail section and highlight the Large mail option.
4. By default, the large mail threshold is 100 KB. To change it, click on the Choose… button under Customize Search Folder.
5. This opens the Mail Size window.
6. In the Show mail greater than a box, type or use the mini arrow buttons to reach your desired value (e.g., 5000 KB/5 MB) and click OK
   • Note: All values are calculated in KB.
7. Press OK again on the New Search Folder box. This creates a folder with all the filtered content.

Every new email that arrives in your mailbox will be checked against this threshold and, if it exceeds the limit, will appear in the folder. This speeds up the search for large, space-occupying emails and facilitates the cleanup process.
Note: The maximum acceptable value in the Show mail greater than the box is 99999 KB (~100MB). However, avoid setting such a high limit as emails rarely reach that size. Moreover, Microsoft has a 36 MB limit on incoming/outgoing emails, making a higher limit impractical.

Use Mailbox Cleanup Tool for Size Reduction in Outlook 365

Creating folders won’t reduce your mailbox size. Outlook has other tools for that purpose. Let’s explore them:

1. Click on File in the main menu.
2. Expand Tools and select Mailbox Cleanup…
3. View the current mailbox size.
4. Filter items on specified criteria.
5. Trigger AutoArchive.

Manage Attachments for a Smaller Mailbox

Your inbox should not be a dumping ground for attachments. Email attachments contribute significantly to mailbox storage usage. Removing attachments and storing them elsewhere can greatly reduce mailbox size. Here’s how:

• Sort emails by size or use the “with attachments” filter.
• Select the email containing the attachment.
• Right-click on the attachment and select Save All Attachments…
• Click OK on the confirmation pop-up screen.
• Choose a location to save the attachments and click OK.
• Go back to Outlook, right-click the email, and select Remove Attachment.

Note: Microsoft Cloud storage is shared between Outlook and OneDrive. To completely remove attachments from Outlook and free up cloud storage, you may need to remove them from OneDrive.

Cleaning Up Redundant Conversations

Long email threads can accumulate a lot of unnecessary data. Outlook has a “Clean Up Conversation” feature to address this:

1. Find the Clean Up icon (a trash bin with a cross sign) in the Delete subsection of the Home tab.
2. Click on the icon to access the feature.
3. You have three options:
   • Clean-Up Conversation: Removes redundant parts of the current email thread.
   • Clean Up Folder: Removes redundant parts of conversations in the selected folder.
   • Clean Up Folder and Sub Folders: Applies the cleanup operation to the selected folder and all subfolders.
4. While setting the cleanup, you can specify certain messages that should be ignored during the data cleanup operation.

Admin-Level Mailbox Size Reduction via PowerShell

Administrators can use PowerShell to reduce mailbox size at the organizational level. While a GUI method exists, PowerShell is quicker and more customizable.

• Connect to Exchange Online PowerShell.
• Check the current mailbox usage with the following command:

Get-MailboxStatistics -Identity  | Format-List DisplayName,ItemCount,TotalItemSize,StorageLimitStatus,UsedPercentage

• Run the Set-Mailbox cmdlet with appropriate quota parameters to customize/reduce size:

Get-Mailbox | Set-Mailbox -ProhibitSendQuota  -ProhibitSendReceiveQuota  -IssueWarningQuota

You can use filters with Get-Mailbox or Get-User to target specific users.

Auto-Archiving Emails

You can trigger an AutoArchive option that periodically moves emails that have been in your mailbox for a set time period to a separate PST file. This reduces the mailbox size.
Note: This feature is only available with select premium M365 plans. Check with your organization before proceeding.

• Open MS Outlook Classic.
• Go to File > Options > Advanced.
• Locate the AutoArchive section and click the AutoArchive Settings button.
• Set the options for your AutoArchive, including frequency, whether it should be announced/unannounced, how it should handle deleted items, whether to view/hide folder, qualification time(older than value), and where to send the archived items.

Shrinking the PST File

• If you maintain an offline copy of emails in a PST file, you can reduce its size by compressing it:
• Go to the File tab in Outlook.
• Select the Account Settings option in the Account Settings section.
• Go to the Data Files tab, select a PST, and click on Settings.
• In the Outlook Data File window, click the Compact Now option.
• Once done, press OK.

Conclusion

Now that you know how to reduce mailbox size in Outlook 365, excess email baggage should no longer be an issue. We’ve provided various approaches through the classic client, web portal, and even a PowerShell script for admins.
No matter what your use case is, regularly practicing these methods is essential while using Outlook 365. Keep this guide handy for future reference.

Frequently Asked Questions

Q. How can I easily identify emails with specific keywords or attachments (like for e.g., “bill,” “.pdf,” etc) to delete or archive their contents?
A. Outlook has a built-in search bar you can combine it with advanced filters like:
“bill” hasattachments:yes This filter here will look for all emails that contain the word bill and an attachment file.
If you maintain a folder-level distribution, then you can search with those specific folders as well. Some add-ins may enhance the searchability, so you can try them as well.

Q. What are the best file naming conventions for attachments that I store outside of Outlook, to ensure easy retrieval later?
A. Use a consistent yet descriptive naming convention that is applicable to a large range of file formats.
Include a date, relevant keywords, and separate words via underscore(_) or hashes (-). Remove any special characters, and include the file extension in the viewing format.

Q. Is it possible to build a layered filter of multiple criteria like from a specific sender, before a specific date, etc?
A. Yes, make a folder as usual
Right Click on the Search Folder
Click Customize this Search Folder
Click Criteria
Within the Criteria window, you can set multiple fields for emails to be filtered by