Outlook - Mailbox Cleanup on Windows

How big is my current mailbox?

Before starting with cleaning up, you’ll first need to get an idea of where the bulk of the data is at. To get an overview of the total size of your mailbox and each individual folder you can open the Folder Size dialog;

Open the Mailbox Cleanup dialog;
- Outlook 2016 / 2019 / Office 365
  File-> section Info-> Tools-> Mailbox Cleanup…
Click the button: View Mailbox Size…
In the window that is being displayed now, you’ll see an overview of your mailbox folders and their sizes.
Overview of folders in the mailbox and their sizes.

When connecting to Exchange, you’ll see a Local Data and a Server Data tab. You only have to look at the Local Data tab; cleanup changes you make will automatically take place on the server as well.

How big are the e-mails?

To see how big the individual messages are, you might need to add the “Size” column to your view. To verify this, or to add this column;

Open the “Customize View” dialog;
- Office 365 (Classic Ribbon)
  View-> View Settings
- Office 365 (Single Line Ribbon)
  View-> Current View-> View Settings…
In the Customize View dialog press the “Columns…” or “Fields…” button
If the “Size” field is not visible yet in the right list named “Show these fields in this order”, select it in the left list name “Available fields” and press the Add button.
Press OK to leave the open windows.

You’ll now see the “Size” column in your Outlook view when you are using the Single Line Layout.

View-> Change View-> Single

Cleaning up

So now that we’ve got a good view of how big the folders are and how we can identify large mails, it’s time to clean up.
I always start with the easiest folders and the folders where I can delete the most. I will handle the clean up process in the following order;

Deleted Items

This one is easy; you wanted to get rid of it? Well, do get rid of it! Right click the Deleted Items folder and choose “Empty Folder”.

Junk E-Mail

Unfortunately, nowadays everybody needs a Junk Email folder. Cleaning this folder is nearly as quickly as emptying the Deleted Items folder.

Scroll through this folder one last time to make sure that there aren’t any still needed e-mails in it. If so, you can move these to the appropriate folder. Once you’re done, you can easily empty the folder by right clicking on the folder and choose “Empty Folder”.

Outlook has a “Delete All” command for any folder.

Large messages need to go first

Now that the trash cans are empty, we’ll continue the cleanup process with first finding the largest chunks in the mailbox. In most cases you’ll find that 20% of your mailbox items account for 80% of the size. You can find these large chunks by using a Search Folder called “Large mail”. If you do not have this folder, you can create it via;

Office 365 (Classic Ribbon)
tab Folder-> button New Search Folder
All Outlook versions
Right click on “Search Folders” in the Folder Pane-> New Search Folder…
Keyboard shortcut
CTRL+SHIFT+P

In the dialog that pops-up, select “Large mail” and set the “Show mail at least this large” option to 5120 KB (which is 5 MB). If not a whole lot of messages are found, you can reduce this to 3072 KB (3 MB).

As an alternative to Search Folders, you can also use the Mailbox Cleanup Tool.
File-> section Info-> Tools-> Mailbox Cleanup

Now that you have found the largest items in your mailbox, you basically have 4 options;

You don’t need the message or the attachment;
Use SHIFT+DELETE to delete the message at once
You still need the message but don’t need the attachment anymore;
Double click on the e-mail to open it-> right click the attachment and choose “Remove”. Save and close the message
You don’t need the message but you still need the attachment;
Right click the attachment-> choose Save As and save the attachment. Then use SHIFT+DELETE to get rid of the message
You still need the message and the attachment;
You can leave it then although this doesn’t help cleaning up.