What do I do when I receive the error message "Your mailbox has exceeded one or more size limits set by your administrator?"

There are mailbox size limits. You will need to archive some of your messages on to your personal folder. By default, your outlook should be configured to automatically download mails onto a personal folder on your machine unless special instruction were provided not to.
If this is the case, due to the mailbox limits on the server, you are adviced to frequently archive your mails onto your personal folders to free up space on your mailbox.