First let me point out what the Quota is on your email system. There is a mechanism that Bluehost doesn't use/doesn't know how to use or is broken. I got several responses from them stating one or the other.
The quota utility is suppose to Reset every month/period/periodically. This is a limit YOU set for your people to limit the amount of mail they receive so they aren't overwhelmed with data/email. You can set it to about however much you want OR (here is the kicker) set it to 0 to disable.
The problem here is that the quota system keeps building every email you receive takes away from that credit until you get a message. Since it doesn't reset, you call, they tell you that you must delete some message (and it is empty) or delete a file because it is reporting incorrectly.
If you get the message, just set the amount to 0 and send a letter to BH telling them to fix the quota system.
This was in response to several older threads that weren't answered correctly. I searched and didn't find a solution myself.