Reducing and Maintaining Microsoft 365 Mailbox Size

This article provides multiple methods and strategies to reduce and maintain a user’s Microsoft 365 (M365) mailbox size. It includes instructions for both Outlook on the web and the Outlook desktop client on Windows and macOS.

Why Mailbox Size Management is Important

Performance Improvement: A smaller mailbox size improves the speed and performance of Outlook, reducing load times and improving responsiveness.
Storage Optimization: Office 365 provides a limited amount of storage for each mailbox. Managing the size ensures you stay within your allocated storage limits.
Compliance and Security: Proper email retention and archiving practices ensure compliance with industry regulations and company policies.
Enhanced Searchability: Smaller mailboxes make it easier to search for and retrieve specific emails.
Backup and Recovery: Smaller mailboxes are easier and quicker to back up, ensuring important emails are preserved.

Strategies to reduce mailbox size

Delete or archive unneeded messages with large attachments.
Delete or archive unneeded old messages.
Enable Auto Archive.

Delete or Archive Unneeded Messages with Large Attachments

Outlook on the Web:

Open Outlook Web Access and sign in.
Select the folder you want to search (e.g., Inbox, Sent Items, etc.).
Change the default option from “Conversations by Date” to “Conversations by Size”.
Once selected, the option next to “Arrange by Size” should change to "Largest on top".
Select the email you would like to delete or archive.
Click Delete or Archive.

Outlook Desktop App for Windows:

Open Outlook and go to the folder you want to search (e.g., Inbox, Sent Items, etc.).
Click on the “View” tab in the ribbon.
Click on “View Settings” and then “Columns”.
In the “Select available columns from” drop down, choose “Size”.
Click “Add” and then “OK”.
Your mailbox will now display a “Size” column.
Click on the “Home” tab in the ribbon.
Click on the “Size” column header to sort your mailbox by size, with the largest emails at the top.
Select the email you would like to delete or archive.
Click Delete or Archive from the ribbon.

Outlook Desktop App for MacOS:

From the View menu, choose Reading Pane > Hidden. This displays your content like a spreadsheet.
Like a spreadsheet, you can click the headers to order the entries ascending or descending by a particular column.
If Size does not display, right-click the header and choose Size from the options.
You can drag the columns around to change their order.
Select the email you would like to delete or archive.
Click Delete or Archive from the ribbon.

Please note that these steps may vary slightly depending on the version of Outlook you are using.

Identify Your oldest Email

Outlook on the Web:

Open Outlook Web Access and sign in.
Select the folder you want to search (e.g., Inbox, Sent Items, etc.).
Click the hamburger menu from the folder view.
Click Sort then click “Oldest on top”
This folder is now sorted oldest to newest.
Select the email you would like to delete or archive.
Click Delete or Archive.

Outlook Desktop App for Windows:

Open Outlook and go to the folder you want to search (e.g., Inbox, Sent Items, etc.).
Click on the “View” tab in the ribbon.
Click on “View Settings” and then “Columns”.
In the “Select available columns from” drop down, choose “Received”.
Click “Add” and then “OK”.
Your mailbox will now display a “Received” column. Click on the “Home” tab in the ribbon.
Click on the “Size” column header to sort your mailbox by size, with the largest emails at the top.
Select the email you would like to delete or archive.
Click Delete or Archive from the ribbon.

Outlook Desktop App for MacOS:

From the View menu, choose Reading Pane > Hidden. This displays your content like a spreadsheet.
Like a spreadsheet, you can click the headers to order the entries ascending or descending by a particular column.
If Received does not display, right-click the header and choose Received from the options.
You can drag the columns around to change their order.
Select the email you would like to delete or archive.
Click Delete or Archive from the ribbon.

Please note that these steps may vary slightly depending on the version of Outlook you are using.

Enable Auto-Archive

Outlook for the Web:

Open Outlook Web Access and sign in.
Go into your mail account settings.
Navigate to Mail, Rules, and click Add new rule.
Name your rule and complete the Add a condition section.
Choose to Archive in the Add an action section, then click Save.

Outlook App on Windows:

Open Outlook and click on the File tab.
Select Options from the menu.
In the Options window, go to Advanced.
Under the Auto-Archive section, click on Auto-Archive Settings.
Check the box that says Run Auto-Archive every [n] days and specify how often to run Auto-Archive.
Choose any other options you want, such as having Outlook delete old items instead of archiving them.
Click OK to save the settings.

Outlook App on MacOS:

The Auto-Archive feature is not natively available in Outlook for MacOS. However, you can manually move or delete old items, or set up rules to move or delete items. If you have access to a Windows machine, you can set up Auto-Archive there, and the settings will apply to your account on the MacOS machine as well.

Please note that these steps might vary slightly depending on the version of Outlook you are using.