One possibility could by e-mail inbox size. Often this is included in the size count.
Check your mailbox via webmail (not your mail software) to check the actual contents of your mailbox. If this is the case, check the settings of your mail software. Often there's an option like "leave a copy on the server" (which is fine) accompanied by a "remove from server after X days".
The latter might not be checked or set. If not, set it. Something like 10 days should do, but 30 days is an option too (but, requiring more space since you can gather mails for a month).
If you delete files, you create room for new mails. The error is gone for a few minutes while new mails which where in the queue drop in, and your mailbox/disk is full again. This is a background process.