@WibbleMe
Except that Google do indeed do just that - or at least, they were doing so when they were the outsourced mail provider for Virgin Media (for email from one [!] of several identically-configured domains on a shared mail server).
Recently, Hotmail have reverted to rejecting email rather than silently swallowing it, so that I do at least know that messages have not been delivered (I'm still peeved that we're being blacklisted without good cause).