[exim] Spam prevention methodics

I can be wrong to ask it here, but

i heard there a lot of custom spam prevention methodics used in MTA during SMTP session time
currently i'm interested in a one when mail is prohibited in case of "too many not found recipients in TO header."
Is it reasonable to use it in production and does not violate netiquette? Or there are some more methodics (where to read?).

Ivan