Limit number of recipients send out external once

Hi all,

I have Exchange server 2013 CU7, I want to limit number of recipients when my users send email to external/internet.
For example:
If my users send email to more than 50 external recipients once, they will receive error "bla bla"

Limitation is not apply when my users send email to internal recipients"
or external sender send email to internal recipients

How can I do it ? (which send connector , receive connector , transport config , transport service , transport server)

Thank for your help.

Jack