There is just one very, very small detail someone missed out somewhere: this only seems to work with Gmail's webmail interface (including their iPhone/iPod webmail interface), as far as I can tell so far. If one sends their Gmail or Gmail-hosted domain e-mail messages to Gmail via authenticated SMTP, Gmail does not then on-send it via that external SMTP server; the message is sent directly from Gmail. That and the address in the
Sender: header you were hoping to completely hide ends up in the
Return-Path: header instead.
Oh well. It is a start, somewhat.