Mailbox Cleanup in Outlook is a set of tools designed to get rid of rubbish from your mailbox. You can use it to review the size of your mailbox, clean out items automatically by size and date, and perform a few other functions. Lets go through them and take a look.

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First, you need to open the tool. Head to File > Tools and click Mailbox Cleanup.

There are some tools available, and we’ll start by looking at the amount of data in your mailbox.

Review the Size of Your Mailbox

Your Outlook mailbox is made up of folders—Inbox, Sent Items, Archive, and so on—and knowing the size of the folders helps you see where all your emails are. To find out, click “View Mailbox Size.”

This opens up the Folder Size window, which shows you the size of each folder. By default, it opens up in the Local Data tab. Local data is the data that is stored on your computer in your .pst and.ost files.

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The columns show the folder’s name, size, and total size (which is the size including any sub-folders). The best part of this view is that it shows you folders you might not normally consider, like the calendar. If you’ve got several years of appointments and meetings requests, you could easily have a decent chunk of kilobytes in the calendar which you can clean up by deleting old events.

If you want to see the size of the folders on the mail server, click the “Server Data” tab. The first time you open this tab, it might take a little while for the data to populate. This is because Outlook is contacting the Exchange server, asking for the data on your files (which is calculated on the fly), and then downloading and displaying the results.

Every subsequent time you open this tab, the data retrieval will be much quicker, since Exchange only has to perform a difference analysis and most folders don’t change size that much. Once the tab is loaded, you’ll see the folder sizes on the Exchange server. This might not be the same as the size in the Local Data tab because Outlook doesn’t necessarily download all messages to your computer.

For example, on my Local Data tab the Total Size of the Archive folder is 116805 kb, but on the Server Data tab, the Total Size is 363399 kb—a difference of 246594 kb (265 MB). That’s a lot of mail not visible in the local data. What’s going on?

By default, Outlook only keeps a certain amount of mail on your local client to make sure your email doesn’t take up too much space. On computers with a hard disk bigger than 64gb (which is most of them nowadays), Outlook will only keep emails from the last year on your computer. Older mails are kept on the email server, and you can access them. You can download the older mails, and change this default if you want to. Bear in mind that you can change the default so that it downloads less mail as well, so if you’re struggling for space, you might want to change the default from 1 year to 6 months (or less).

(Note: This only affects accounts that use a Microsoft Exchange server, like Hotmail, live.com, O365, and some corporate accounts. If you’re using Outlook to download mail from other providers like Google or Yahoo this setting is ignored and all mail is downloaded.)

Now that you know how much data your folders contain, you can prioritize which ones you clean up.

Searching for Old or Large Mail

If you’ve got mail in lots of different folders, you can search across your whole mailbox for older or larger mails.

This is very useful if you want to find mails with large attachments that you can either delete, or move from Outlook to a hard drive or cloud service. As an example, if you want to find all mails over 5 MB, you could en change the “Find items larger than” field to 5000 kb and then click the “Find” button.

The standard Outlook search panel comes back with a list of mails that are at least 5 MB.

These results only return emails, but you can change the “Look” field to any specific type of file if you want to cast a wider net.

From the search results, you can open each of the emails and delete them or remove the attachments. Don’t forget to save the attachments before deleting or removing them if you need to save them.

Kicking off an AutoArchive

You can set autoarchive parameters for every individual folder in Outlook, something we’ve covered before as part of a wider look at keeping Outlook running smoothly. If you’ve set up autoarchive rules, you can kick them off in the Mailbox Cleanup by clicking the “AutoArchive” button.

This process swings through all of the folders in Outlook and applies any auto-archive rules you’ve set up. If you haven’t set any up then you might be worried about hitting this button by accident, but don’t worry—the default autoarchive rule is to do nothing.

View and Empty the Deleted Items

If you like to “soft delete” your mail by moving them to the Deleted Items folder and you rarely empty it, this option is for you.

Note: Some people like to use their Deleted Items folder as a sort of archive, tossing things in there and never emptying the folder. This is not a good idea, because there are plenty of ways you can empty the Deleted Items folder too easily. Instead, if you need to keep a message, set up an archive!

Click “View Deleted Items Size” to see how much space your deleted items are taking up, then take a deep breath and click “Empty.”

This will delete all items from the Deleted Items folder. If that scares you then by all means take a moment to move any important mails into your Archive first, but you really should empty your deleted items regularly and not use it as an archive.

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View and Delete Conflict Mails

The final option in Mailbox Cleanup is to delete conflict messages. These are emails that Outlook thinks conflict with an existing email because they have the same header information. Normally this is because of temporary syncing issues with the Exchange server, and you should be able to delete these emails without a problem. To see if you’ve got any emails in the Conflict folder, click the”View Conflicts Size” button to see a filtered version of the Folder Sizes window.

If there are any emails there, close the Folder Size window and then click the “Delete” button in the Mailbox Cleanup window.

That’s the Mailbox Cleanup tool. It’s not particularly flashy, but it’s effective and useful.