When some teachers reach their storage quota in their school email, they have the misconception that deleting a bunch of old emails will make more room. Actually, most day-to-day emails don't take up that much room. Attachments are what cause the problem, especially images, presentations, movies, and PDF files. Here's how to get rid of your memory hogs.
Your Inbox is usually sorted as "Conversations by Date." Change it to sort by "Size"
Then delete messages in the "Enormous" and "Huge" category. If you need to save the attachments, do so before deleting the message so that you have a copy.
Once you are finished, go back to sorting by "Conversations by Date," and you're done!