When someone sends you an email, the message is stored on your web space waiting for you to collect it.
Usually when you check your email, it will remove any messages once you've collected them and this keeps your mailbox size small.
However your email programme may be set to leave a copy of the message on the server.
This is useful when you want to check email on your iPhone or Blackberry while you're out of the office, so that it also downloads the email when you get back to your desk.
However you shouldn't leave all your email on the server or you'll exceed the amount of space that you're allocated, and then you won't be able to receive any more emails.
We'll try to warn you before this happens, but you'll need to make a change to your email programme to remove emails from the server when using your office computer.
Depending on the email programme, this setting may be in a different place. We've listed the most common email programmes on the right.