That sounds like most SMTP servers - they just reject what they don't understand. The wrinkle here that might be, in the opinion of some, worth noting is that

That sounds like most SMTP servers - they just reject what they don't understand. The wrinkle here that might be, in the opinion of some, worth noting is that this gives SMTP servers something they understand fully. The HTTP server has been convinced to send perfectly well-formed and valid SMTP commands.