Originally Posted by tweakservers
Check the option "Discard emails for users who have exceeded their quota instead of keeping them in the queue." in WHM > Exim Configuration Editor so that if there is a user mail box is full, the bounce will be discarded by your server.
The way I understand this option, this only works in regards to an account's overall quota and not individual mailbox quota. Secondly, all this option does is prevents messages intended for accounts that are over their quota from being stored in the mail queue. Typically, a spammer will send a message and not use a valid return address. When this message reaches the server and the above option is enabled, the message is not attempted to be delivered, and a message back to the sender is created telling the sender that the account is over its quota. If the return address on the message is invalid, then you are stuck with a bounce message in your mail queue trying to be sent to an address that does not exist. This I believe is what the original poster is referring to.
My solution would be to disabled mailbox quotas or atleast set the quotas to unlimited for mailboxes that have reached their quotas. This is why I'm not entirely sure what the point of a mailbox quota system is. You are either checking a mailbox or you are not.
If an overall account is over its quota, make sure its default box is set to :fail: and then disable all mail accounts on the account, so that messages for the domain are immediately rejected.