Something else to keep in mind is that treating email as if it were postal is plain bad business.
A mistake here can destroys your business brand. It can get your company's domain blacklisted with ISP's. It can land you a hefty fine from regulators, if anyone pursues the matter against you.
So, a better alternative, whenever unsure, would be to use postal for prospecting with strangers while email for dealing with contacts who already know you and have a relationship with you.