At my domain I want to implement a feature for restricting senders for some user accounts usr1,usr2, ....usrn,usrn+1 for example the user1 and usr2 should be able to send at a time only to 5 recipients not more, usr3 and usr4 can send to 10 and usrn can send to 1000. if user send more mails should not be sent an should return with a warning message to the sender.

Your input would be appreciated?