That means you are using Exchange as your mail server (be sure to
mention such important details). You are allocated a fixed amount of
disk space for your mailbox. You've been saving way too much in your
mailbox. Delete the garbage and start using AutoArchive to move some of
it out. If you absolutely must need all of your items to remain in your
Exchange mailbox (and prove it), contact your Exchange admin to have
them increase the size of your mailbox.
