You are reading this article becasue your qouta has gone over the limit but when you check the disk usage you see an abnormally high amount of MB's being shown in "Other Usage "section.
This issue is usually caused by the .Trash located in the mail system, so if you have deleted alot of emails, then it does not get permanently deleted but gets assigned to "Other Usage".
To purge the .Trash you need to login to webmail and go to the trash folder and permanently delete all the deleted emails.