Your E-Mail Quota
Many accounts have a disk quota -- how much disk
  space you can use to store your files (including email folders)
  in your account. If you use more than this space, you are said
  to have "gone over" or "exceeded" your quota.
  If that happens, various
  things may not work correctly, or you may not see new mail
  messages until you return to within your quota allocation.
It is important to monitor your quota and make sure you do
  not exceed it for proper functioning of your account and
  protection of your files.