In short, I believe you can't be sure. Every service I know suffers the problem of black-box, uncontrolled, unknowable spam filters at the destination, which i

In short, I believe you can't be sure. Every service I know suffers the problem of black-box, uncontrolled, unknowable spam filters at the destination, which is why web confirmation pages always say something like: "If the email hasn't arrived within an hour or so, check your spam bin." It can be mitigated by "doing things properly" but you can never be sure. Make sure your email is contentful and meaningful. Anecdata: I use sendmail pretty much directly from my shared-hosting service and things very rarely go missing.