How to reduce mailbox size in Outlook?

Lots of email messages will slow down the Outlook’s performance. In this tutorial, it will introduce several methods for reducing mailbox size to enhance the Outlook’s productivity,

Note: before trying the solutions provided, please make sure that you have taken a full backup of the Outlook PST files.

All methods provided in this tutorial are operated in Outlook 2021, there may be some differences in different Outlook versions.

Find and delete large emails and attachments

Method 1 Sort emails by size

Method 2 Search email by size

Using Mailbox Cleanup

Empty Deleted Items

Move old mails to Archive file

Compress the Outlook data file

Archive older Outlook data file

Clean Up redundant messages

Find and delete large emails and attachments

There are two methods can help you quickly find the large emails and attachments.

Method 1 Sort emails by size

In outlook, generally, you sort emails by received date, actually, you also can sort emails by their sizes.

2. Then go to the By Date drop-down list, choose Size.

Now all emails in the folder are sorted by size, and Outlook auto groups emails by size to 4 groups.

Then you can remove the large emails.

Method 2 Search email by size

1. In an account folder list, scroll to Search Folder, and right click it to click New Search Folder.

2. Then in the New Search Folder dialog, scroll to Organizing Mail section and click Large mail, then click Choose to pop the Mail Size dialog, and specify the small size at least that you want to find mails based on. Click OK > OK to close the dialogs.

Now you can see a new folder is created, and all mails whose sizes are greater than 50 kb are listed in this new folder, you can browse them and remove the mails that you do not need any more.

Using Mailbox Cleanup

The Mailbox Cleanup tool is very useful for reducing mailbox size. With it, you can empty the Deleted Items folder to release space, and also can click Auto Archive to move older mails to the archive file.

Click File tab, then click Tools > Mailbox Cleanup..

Empty Deleted Items

Click View Deleted Items Size to see the size of the Deleted Items folder, then go back to click Empty to permanently remove the items in the Deleted Items folder.

Move old mails to Archive file

Click Auto Archive button to move old items to the archive files on the current computer.

Compress the Outlook data file

In Outlook, you can manually compress the Outlook data (PST) file.

1. Click File tab, then click Account Settings > Account Settings.

2. In the Account Settings dialog, click Data File tab, then choose one account that you want to compress its data file, click Settings.

3. In the Outlook Data File dialog, click Compact Now. Click OK.

Archive older Outlook data file

1. Click File > Options, and in the Options window, click Advanced tab in the left pane, then click AutoArchive Settings.

2. In the AutoArchive dialog, check Run AutoArchive every n days, and then specify this option and other options as you need such as Clean out items than. Then click OK > OK.

Clean Up redundant messages

Using the Clean Up feature group to remove the redundant messages in selected conversations, folders or subfolders

Select the folders or the conversations, and click Home tab, and in the Delete group, choose one feature from the drop-down list of Clean Up.