I don't think you should hold back..send those emails, it might make you feel better!!
I have given myself a second job, which consists of emailing people who make me mad by either selling tiny yorkies for breeding, or people who i believe to be puppymills or BYB etc....go on, go for it, send the emails and see how good you feel after!! (but be prepared to get some abuse back sometimes!)
If it makes one person stop and think twice about what they are doing, it will have been worthwhile.