I would personally never get into a side project for anything that requires decent fraud prevention (email, telecom, payments, etc.). I have worked at medium-sized startups in these areas struggle to keep their heads above water on fraud prevention even with well-staffed and well-paid teams of experts.

It's a never ending battle where you cannot win, you just manage to not lose so badly that lawyers and federal regulators pound you in the behind or users abandon your platform.