[Exim] <> sender rejection on multiple recipients

This may be a constructed memory, but I dimly remember some discussion
about limiting the number of recipients of a message if the sender was
<>, as a means to cut down spam, since non-delivery notifications are
normally only sent to the sender of a message, and not to multiple
persons. (Although I can see some systems also sending to postmaster) -

Anyway, what I think would work is at SMTP-time, if the sender is <>,
and there are more than a configurable number of recipients, then
reject the message after the DATA section, with an appropriate log
entry - "Rejected - Too many recipients (x) for error message"

Is this something that someone is already doing?