How to Reduce the Size of a Full or Nearly Full Outlook Mailbox


When your mailbox reaches its upper limit, your administrator might start limiting functionality. When you receive a warning that your mailbox is full or near full, the first thing to do is to verify that the warning is true and accurate. Many phishing messages claim that your mailbox is full or getting full in an attempt to get you to provide your log-in credentials or other sensitive information. To see a list of known phishing emails reported on campus, please see the CSULB Phish Bowl.


How to check if Mailbox is full

  1. You can verify the size of your mailbox by clicking "File" in Outlook's menu

    File option in Outlook client
  2. Then look at the graph under "Mailbox Cleanup".

    Mailbox Cleanup in Outlook
  3. If you confirm that your mailbox is full or nearly full (the bar on the graph will turn red when it reaches a certain high threshold near full), then proceed to cleaning up your messages in order to reduce the mailbox size. There is a really simple and efficient method to do this that will help you reduce your mailbox size with minimal effort. Follow the procedure below to implement it:

How to Reduce Mailbox size

The most efficient way to reduce your mailbox size is to tackle the largest messages in your mailbox (those with large attachments). Any other method will have you spinning your wheels with not much to show for your efforts. This is because the largest messages in your mailbox can be equivalent to thousands of regular messages that don't have large attachments.

The easiest way to review these large messages is to set up a search folder in a few simple steps:

  1. In your list of folders, scroll down and right-click on "Search Folders".
  2. Click on "New Search Folder".

    Search Folder, right-clicked New Search Folder
  3. Scroll down the list and select "Large mail" located under "Organizing Mail". Then click "Choose..."

    New Search Folder options
  4. Type "5000" in the box. This will include messages larger than 5 megabytes in the search folder, which are considered "very large". Click "OK", and then click "OK" again.

    Mail size
  5. If you now expand the "Search Folders" container, you will see a folder named "Larger Than 5000 KB". This folder will continue to be automatically updated to show all messages larger than 5 MB that are in your mailbox, sorted by size, starting with the largest ones at the top.

    New Folder, Larger than 5000 KB

How to Clear Large Messages or Attachments

Whenever your mailbox is getting full, open the search folder that you just created and open the messages one-by-one in order of size.

If you no longer need the message or its contents, either delete the message or delete its attachments (you can save the attachments to disk beforehand by right-clicking on them and clicking "Save As" or "Save All Attachments"). Repeat this procedure for all other messages in this Search Folder until you have cleared enough space in your mailbox.

If you delete the messages themselves, don't forget to go to your "Deleted Items" folder afterward to empty it or delete the large messages from there as well. You may also empty the entire "Deleted Items" folder by right-clicking it and clicking "Empty Folder".

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Article ID: 58845
Wed 8/1/18 8:55 AM
Thu 2/3/22 3:07 PM