Mailbox cleanup in Outlook consists of a set of tools that you can use to remove garbage from your mailbox. It allows you to check the size of your mailbox, automatically clean up items by size and date, and perform some other functions. Let's go through it and take a look.
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First, you need to open the tool. Go to File> Tools and click Mailbox Cleanup.
There are a few tools available, and we'll first look at the amount of data in your mailbox.
Check the size of your mailbox
Your Outlook mailbox consists of folders – Inbox, Sent Items, Archive, and so on – and if you know the size of the folders, you can see where all your emails are are located. To find out, click on "Show PO Box size".
This opens the Folder Size window, which displays the size of each folder. By default, it opens on the Local Data tab. Local data is the data stored in your .pst and .ost files on your computer.
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The columns display the name, size, and total size of the folder (this is the size of all subfolders). The best part of this view is that you are viewing folders that you do not normally consider, such as: For example, the calendar. If you have appointments and meeting requests for several years, you can easily save a neat kilobyte block in the calendar that you can delete by deleting old events.
If you want to see the size of the folders on the calendar mail server, click on the tab "Server data". When you open this tab for the first time, it may take a while for the data to fill. This is because Outlook contacts the Exchange server and asks for the data of your files (which are calculated in an instant) and then downloads and displays the results.
Each time you open this tab Data retrieval becomes much faster because Exchange only needs to perform a difference analysis and most folders do not resize so much. Once the tab is loaded, you will see the folder sizes on the Exchange server. This size may not match the size on the Local Data tab because Outlook does not necessarily download all the messages to your computer.
For example, on the Local Data tab, the total folder size of the archive is 116805 KB, but on the Server Data tab, the total size is 363399 KB – a difference of 246594 KB (265 MB). This is a lot of mail that is not visible in the local data. What's happening?
By default, Outlook only saves a certain number of emails to your local client to make sure your emails do not consume too much space. On computers with a hard disk that is larger than 64 GB (most of them today), Outlook only saves emails from last year on your computer. Older emails are kept on the email server and you can access them. You can download the older mails and change this standard if you want. Keep in mind that you can change the default setting to download fewer emails. So if you're struggling for storage space, you should change the default value from 1 year to 6 months (or less).
(Note: This only applies to accounts that use a Microsoft Exchange server, such as Hotmail, live.com, O365, and some corporate accounts. If you use Outlook to download emails from other providers, such as Google or Yahoo, set this setting is ignored and all emails are downloaded.)
Now that you know how much data is in your folders, you can prioritize what you clean up.
Searching for old or large mail
If you have mail Many different folders allow you to search your entire mailbox for older or larger mail.
This is very useful if you want to find emails with large attachments that you can either delete or from which you can continue Outlook to a disk or cloud service. For example, if you want to find all e-mails over 5 MB, you can change the "Find items larger than" field to 5000 kb, and then click the "Search" button.
The default The Outlook search box displays with a list of emails that are at least 5 MB in size.
These results only return emails. However, you can change the Search field to a specific file type
From the search results, you can open and delete each email, or remove the attachments. Remember to save the attachments before deleting or removing them if you need to save them.
Starting an AutoArchive
You can set auto-archive parameters for each individual folder in Outlook, which we discussed earlier as part of a broader view of the smooth running of Outlook. If you have set up automatic archiving rules, you can disable them in the Mailbox Cleanup by clicking the AutoArchive button.
This process iterates through all the folders in Outlook and applies all the automatic archive rules that you have set up. If you have not made any settings yet, you may be concerned that you accidentally press this button. But do not worry – the automatic autoarchive rule is nothing.
Showing and Deleting the Deleted Items
If You Want To "delete" your e-mails by moving them to the Deleted Items folder and rarely deleting them, this option is for you.
Note : Some users like to use their Deleted Items folder as a kind of archive. Throw things in and never empty the folder. This is not a good idea, as there are many ways to empty the Deleted Items folder. If you want to save a message, set up an archive!
Click Show Size of Deleted Items to see how much space your deleted items are taking, take a deep breath, and click Empty. 
This deletes all items from the Deleted Items folder. If this scares you, take a moment to move important mail to your archive first, but you should periodically empty your deleted items and not use as an archive.
LINK:  Automatically flush the deleted items folder when exiting Outlook
Viewing and deleting conflicting mail
The last option in Mailbox Cleanup is to create conflict messages to delete. These are emails that Outlook believes conflict with an existing email because they have the same header information. This is usually due to temporary synchronization issues with the Exchange server, and you should be able to easily delete these emails. To see if you have emails in the conflict folder, click the Show conflict size button to display a filtered version of the Folder sizes window.
If there are emails, click Close on the Mailbox Cleanup window.
The tool for cleaning up mailboxes. It's not very noticeable, but it's effective and useful.