Reject messages (not bouncing) when mailbox full

The problem as I'd see it is that the quota checking in exim is done after the whole email is received (as otherwise you don't know whether that email will put the mailbox over quota) and so you've lost the chance to deny the email at the RCPT stage as you're now past the DATA stage.

Checking on the exim mailing list, it looks like this has been discussed at some length and there are also permissions issue with trying to do this at the RCPT stage in exim too. They say quite clearly that it isn't possible to do this in exim.

One thing to check that you have the following enabled:
WHM > Exim Configuration Editor > Discard emails for users who have exceeded their quota instead of keeping them in the queue

 

Thanks

One thing that I sometimes see is people of have created a Mail account and a Forwarder so that email is forwarded on and delivered to a mailbox. If that mailbox fills up and the Forwarder bounces for some reason (AOL being stupid as usual or their forwarded email address being full!) it can cause email to build in the queue. The fix for that is obviously to delete the mailbox.