Microsoft Outlook is a personal information manager that is included in the Microsoft Office suite. It is most often mainly used as an email application but it also includes a calendar, task manager, contact manager, note taking, journal, and web browser. There are certain steps you should take to maintain a reasonable mailbox size.
This includes making sure deleted items are emptied upon closing out, removing email in sent items and drafts, checking mailbox cleanup features, and archiving. Taking all these steps can assure you that you are getting the most efficient use out of your Microsoft Outlook email client.