Please follow the below guide if you think you need (or have been told to) clear down the size of your mailbox
Mailbox Clean-up:
Open Outlook
Select File > Info > Tools > Mailbox Cleanup
The below menu will appear, here you can choose your preferred settings
Empty Deleted Items
This is a good way to permanently get rid of any un-needed emails, which will therefore free up more space in your inbox.
File > Options > Advanced. Please note this will delete all deleted items every time you close outlook, see below for more info:
