[exim] Max outgoing recipients

I am trying to do something very simple which is to limit the number
of outgoing recipients in a single message, say to 30 max. In other
words, when one of our clients tries to send a message to hundreds of
recipients, he will get an error message saying something like max
number of outgoing recipients is exceeded.

I tried to implement this using: smtp_accept_max_per_connection = 30

but am not certain if it produces the desired effect as I have tested
with max 2 but was still able to send an email with 5 outgoing
recipients. I've tried googling and searching this list but found no
conclusive answer. Another variable which seems related would be
max_rcpts but this is not part of the main configuration parameters,
and perhaps belongs to the smtp transport.

help is much appreciated.