The more email you send and receive, the larger your Outlook mailbox can grow. Large mailboxes can take longer to open and search, and many email providers, including Microsoft 365, limit the maximum size of your mailbox. If you receive a message that your mailbox is over its size limit, deleting messages and folders can help, but if you store your information in an Outlook Data File (.pst) or Offline Outlook Data File (.ost), that data file may not shrink in size automatically.
Note: If you use a POP account, all of your email account information, including your folders, email messages, contacts, calendar, and task information is stored in an Outlook Data File (.pst). If you use an Microsoft 365, Exchange, IMAP, or Outlook.com account, a copy of your mailbox is stored in an Offline Outlook Data File (.ost).
To reduce the size of your Outlook Data File (.pst) or Offline Outlook Data File (.ost), you can manually compact the data file. Select your version of Outlook below to see instructions on how to reduce the size of a .pst or .ost file.
There are several ways to reduce your mailbox size.
You can use the Mailbox Cleanup tool to view the size of your mailbox and individual folders. This tool can also help you search for older or very large messages that you might want to delete or move to an Outlook Data File (.pst).
Often, the cause of a large mailbox is a large Deleted Items folder. The tool provides a button to view the size of your Deleted Items folder and empty your Deleted Items folder.
Occasionally, Outlook will store conflicted items in a folder called Conflicts. Conflicts happen when a copy of an item stored on your email server is out of sync with a copy of an item stored on your local computer. The Mailbox Cleanup tool can delete these conflicts for you.
Run the Mailbox Cleanup tool
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Click the File tab.
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Click Tools > Mailbox Cleanup.
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Select View Mailbox Size, Find items older than, Find items larger than, View Deleted Items Size, Empty Deleted Items folder, View Conflicts Size, or Empty Conflicts to perform your desired task.
Use Conversation Clean Up to remove redundant messages
In a long email thread, there are often numerous messages that contain all of the same information as prior messages. Conversation Clean Up can remove those redundant messages automatically. For more information, see Use Conversation Clean Up to delete redundant messages.
Search for large messages
You can search for all messages larger than a specific size from Outlook's Search box. Click in the Search box and type: messagesize:>5 mb. This will find all messages larger than 5 MB. You can delete some or all of these messages or move them to another Outlook Data File (.pst). For more information on moving items, see Move or copy a message between folders.
If you delete items from an Outlook Data File (.pst) or Offline Outlook Data File (.ost), Outlook automatically compacts the file in the background during idle time, when you’re not using your computer, but Outlook is still running.
Note: Outlook only compacts the file if there is more than 20% of 'white space' (unused space) in the file.
You can also use the following steps to manually start the compact process immediately, which may take several minutes to complete.
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Delete any items that you do not want to keep and empty the Deleted Items folder.
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Click File > Account Settings > Account Settings.
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On the Data Files tab, click the data file that you want to compact, and then click Settings.Note: If you are using an Exchange or Outlook.com email account, you need to additionally click the Advanced tab > Outlook Data File Settings.
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In the Outlook Data File dialog, click Compact Now and then click OK.
: You do not have to exit Outlook after you compact an Outlook Data File.
