When your mailbox reaches 100 GB in size, you are no longer able to send mail. To prevent this from happening, you will want to reduce the size of your email box. 

Error message

In Outlook, the user receives the following error message:

The message store has reached its maximum size. To reduce the amount of data in this message store, select some items that you no longer need, permanently (Shift + Del) delete them.

In Outlook Web App, the user receives the following error message:

The action couldn't be completed. An error occurred on the server.


The solution depend upon how close to the limit you are. You'll need to use a combination of Outlook Web App and the Outlook Desktop Client.

Calculate your mailbox size

  1. Open Outlook and select the File tab
  2. With the Info section selected you will find the size under Mailbox Settings
  3. Your mailbox size will be listed under Mailbox Settings.

Get rid of RSS feeds

It's a good idea to get rid of your RSS feeds. RSS feeds were part of the original upgrade to Outlook from GroupWise, and at one time were useful. Today, though, they just take up a lot of mailbox space, so if you have them, you should delete them. Since they can be quite large, it's easier to do this through the Microsoft 365 Outlook WebApp, rather than through Outlook on your desktop, since the Web App doesn't have to synchronize the changes.

  1. Expand RSS Feeds
  2. For each of the RSS Feeds (Microsoft at Home, Microsoft at Work, MSNBC News, Popular Government Questions from and Updates: News and Features), right-click the folder and choose Delete.
  3. Click OK to the Delete Folder confirmation dialog box.
  4. Once you've deleted all of the RSS Feeds, empty your Deleted Items folder.

Unsubscribe from email you don't need

We all get email sent to us every day from websites or mailing lists we've signed up for. It's a great way to get reminders or alerts about things we're interested in. It's also a great way to increase our mailbox size if we're not vigilant about deleting them when we're done! Unsubscribing from mailing lists that you no longer need means less email coming in, and a smaller mailbox.

Delete what you don't need to keep

This is pretty straightforward. Select the mail or mail folders you want to remove, and then right-click and choose Delete. Don't forget to empty your Deleted Items Folder when you're done. Some places you might want to look for mail you can purge:

  • Drafts - get rid of any drafts you don't need or never sent.
  • Sent items
  • Conversation History
  • Junk E-Mail (you can right-click and Empty this folder)
  • Reminders
  • Restored Items

Focus especially on large items. You can check your folder sizes by opening Outlook on your Windows computer.

  1. Right-click the root of your mailbox (your name).
  2. Click Folder Size.

You can see the size of any subfolder and use that to determine where to focus the most time and effort.

Use Sort to find large emails too, especially the "Size" and "Attachments" sort options.

sort options

Remove attachments from an email and keep the email

Emails with attachments create large emails that take up a large chunk of your mailbox space. Rather than saving those attachments that are emailed to you, save them to your network drives and then remove them from the email. 

  1. Right-click the attachment and choose Save As or Save All Attachments to save the file to a folder on your computer or on your network drive (like P or O). 
  2. Then right-click the attachment and choose Remove Attachment.
    Remove attachment
  3. You'll retain the email message, but the attachment (which you saved elsewhere) will be removed, reducing your mailbox size.

Use Mailbox Cleanup

  1. Open Outlook on your Windows computer.
  2. Click Cleanup Tools in the Mailbox Cleanup section
  3. Here you can do a variety of things, including viewing your Mailbox Size, emptying Deleted items, deleting alternate versions, and finding items that are over a certain size.
    Mailbox Cleanup

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