Mail Box Full?
Email accounts are limited in size so you need to delete emails frequently from:
- your “inbox”,
- your “junk email” folder and
- your “sent" folder and then
- your "deleted items" folder
You can delete all items in the Deleted Items folder by using the right mouse-click and choosing Empty Deleted Items Folder.
If you receive a large attachment from a professor consider saving the attachment to disk and deleting the email.