I think "setrule_mailsend" is a sendmail-ism, not Messaging Server.
If you're looking to limit number of mails per connection, any smart client will automatically send multiple mails, as soon as it finds your limit. Use keywords such as, addrsperjob, filesperjob. Many spam robots don't understand the error message that comes back, and may be affected, though. However, most spam bots send one message per try, anyway.
Not of much use, in my opinion.