It sounds to me like your admins are right. You see, even a single thread can “outrun the mail-server.” Therefore, there's no advantage to having more than one thread or process doing that, plus it's considerably simpler to manage.

If the mail-server is such that it can simply be given an enormous queue of stuff to chomp-through, then just give it the enormous queue all at once and let it chomp-through as it will. If there's an issue of choking-out other mail that needs to go out the same tube, any sort of “simple throttle” should do ... in one process ('cuz you just don't need more).