I see a client on a directadmin ded server here has sent 27k emails in 6 days but directadmin didn't suspend that user! The email limit is already set for the user to 4000/day and Suspend on limit is also set to on!
How can I figure out why directadmin didn't work here?
There is no such a ready feature. I don't recall anybody here who would want the same. So that's up to you to write a script or hire somebody to write a script, which will suspend an user account if daily limit on number of emails will be hit.