Reduce the size of your mailbox and Outlook Data Files (.pst and .ost)

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 Office 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), that data file may not shrink in size automatically.

Note: If you use a POP or IMAP 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 Office 365, Exchange, 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), you can compact the data file. Offline Outlook Data Files (.ost) can't be compacted. See Reduce the size of an Offline Data File (.ost) for more information.

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

  1. Click the File tab.

  2. Click Tools > Mailbox Cleanup.

  3. 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), the file is automatically compacted in the background when you’re not using your computer if Outlook is running. You can also manually start the compact process immediately. It might take several minutes to compact an Outlook Data File (.pst).

  1. Delete any items that you do not want to keep and empty the Deleted Items folder.

  2. Click File >  Account Settings >Account Settings.

  3. On the Data Files tab, click the data file that you want to compact, and then click Settings.

  4. Click the Advanced tab > Outlook Data File Settings.

  5. In the Outlook Data File Settings dialog, click Compact Now and then click OK.

Note: You do not have to exit Outlook after you compact an Outlook Data File (.pst) file.

Many email accounts use an offline Outlook Data File to store data. An offline Outlook Data File (.ost) is very similar to an Outlook Data File (.pst) with one exception. You cannot import from or export to an offline Outlook Data File (.ost).

  1. Click File >  Account Settings > Account Settings.

  2. On the Data Files tab, click the data file that you want to compact, and then click Settings.

  3. Click the Advanced tab > Outlook Data File Settings.

  4. In the Outlook Data File Settings dialog, click Compact Now and then click OK.

Note: You do not have to exit Outlook after you compact an Outlook Data File (.ost) file.

The Mailbox Cleanup tool is used to view the size of your mailbox and individual folders. You can also archive older items to an archive Outlook Data File (.pst) and empty your Deleted Items folder.

  1. Click File.

  2. Click Tools > Mailbox Cleanup.

  3. 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.

If you delete items from an Outlook Data File (.pst), the file is automatically compacted in the background when you’re not using your computer and Outlook is running. You can also manually start the compact command immediately. It might take several minutes to compact an Outlook Data File (.pst).

  1. Click the File tab.

  2. Click Account Settings, and then click Account Settings.

  3. On the Data Files tab, click the data file that you want to compact, and then click Settings.

  4. Click Compact Now.

Note: You do not have to exit Outlook after you compact an Outlook Data File (.pst) file.

Many email accounts use an offline Outlook Data File to store data. An offline Outlook Data File (.ost) is very similar to an Outlook Data File (.pst) with one exception. You cannot import from or export to an offline Outlook Data File (.ost).

  1. Click the File tab.

  2. Click Account Settings, and then click Account Settings.

  3. On the Data Files tab, click the data file that you want to compact, and then click Settings.

  4. Click Compact Now.

Note: You do not have to exit Outlook after you compact an Outlook Data File (.ost) file.

You can use the Mailbox Cleanup tool, which allows you to view the size of your mailbox and individual folders. You can also archive older items to an archive Personal Folders file (.pst) and empty your Deleted Items folder.

  1. On the Tools menu, click Mailbox Cleanup.

  2. 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.

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If you delete items from a Personal Folders file (.pst), the file is automatically compacted in the background when you’re not using your computer and Outlook is running. If you want to immediately reduce the size of the file, you can force a compaction. The procedure to compact the data file might take several minutes.

  1. On the File menu, click Data File Management.

  2. Click the data file that you want to compact, and then click Settings.

  3. Click Compact Now.

Note: You do not have to exit Outlook after you compact a .pst file.

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Reducing the size of the Offline Folder file (.ost) file may improve performance if you have deleted a large quantity of items. This procedure does not delete any items that are saved on the server.

  1. Delete any items that you do not want to keep, and then empty the Deleted Items folder.

  2. On the Tools menu, click Account Settings.

  3. In the list, select Microsoft Exchange Server, and then click Change.

  4. Click More Settings.

  5. Click the Advanced tab, and then click Offline Folder File Settings.

  6. Click Compact Now.

Note: You do not have to exit Outlook after you compact a .ost file.

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