Hi,
Very simple: Every mail box has a capacity.... The capacity can be big or small (some have 3 GBits, some have 5 Mbits..... 7, 10, 20, 40 or more). Now if the user gets & sends e-mails of large volumes and does it frequently he / she will "use" that available capacity..... and than it is "full". By permanently deleting unnecessary files one can re-gain space.
Best regards,