This is CRM (Challenge / Response mail), which is starting to be compared to spam, because it does not scale, it places the burden of spam detection on other, possibly innocent, bystanders. Please search the archives of spam-l, or n.a.n.e.a or n.a.n.e for further information. See also Re: (OT) Fighting spam.

Otherwise, best of luck to you.

--MidLifeXis