This guide will assist you in deleting messages:
Deleting a message is very simple, first select the message you'd like to delete, then click the 'Delete' button:
This action will move the message into the Deleted Items folder.
To fully delete the message, navigate to the Deleted Items folder, and repeat the process from before:
Making sure that this process is done frequently and correctly will ensure that you do not hit the disk space limit on your account.