If your primary device for work is a phone or tablet, you will at some point run into an issue where your inbox has filled up. The reason this happens is that when Emails are deleted out of your mobile device inbox, they are not actually cleared from Outlook.
To fix this, you need to use a phone or a tablet and visit Outlook.com, then do the following:
1. Sign in with your work email and password, then navigate up to Settings and search for/navigate to "Storage"
2. Once there, you can choose to clear emails older than 3, 6, or 12 months of age.
3. Alternatively, if you don't want to bulk-clear your email and you need to Archive certain things so that they aren't deleted, you can go through the emails individually.
One additional tip is that if you're going through them individually, you can highlight one email and then scroll down and shift+click another. This will highlight every email in-between as well, so you can still remove dozens or hundreds of emails at a time without necessarily setting a time span.
For any issues logging into Outlook.com, please contact the Helpdesk.