This was my first response too; there's been a CLI tool (called vrfy) for a while that I've given up on using because it almost always says "send some mail, i'l

This was my first response too; there's been a CLI tool (called vrfy) for a while that I've given up on using because it almost always says "send some mail, i'll try my best". That tends to be the ultimate test, which you can probably already do pretty quickly with a backend for a web form. EDIT: That being said, making a potential customer wait for thorough verification, on top of "spamming" them before they've (technically) bought from you would be unwise; I can see how this would have merit.