Have the user sort by size for one. This will help them find the emails with large attachments. Have them save the attachments to an appropriate space and then delete the emails. We have to educate our users often that the Exchange server is not a file storage server for them. Have them arrange each folder by size to find all large attachments besides those in the main inbox. Sent items is a common one overlooked. We send files we already have saved on our PC, we do not need to keep them in our email as well. Keep in mind they need to EMPTY the deleted items after deleting the emails and that it may take a couple of hours for the Exchange server to recognize their mailbox is smaller again. Train your users to triage their mailboxes BEFORE they exceed their limits to avoid interruption in service.
<i>To remove the message from the outbox folder, click on the message and hit the delete key. If this doesn’t work, highlight the message and hit the Shift+Delete key at the same time.</i>