I don't see why every stage of the journey would need to be different to now - you could leave the certificate verification to the last leg of the journey...
Because the only time you can verify the sender is when something is sent. Otherwise you're just getting the MITM's version of who the sender is/was/should have been. Which would most likely make it easier to spoof because the servers would be tied up focusing on a cert instead of the rest of the message header.
The SPF (Sender Policy Framework) was supposed to be a cure for server validation ... it just never got off the ground. Because as simple as it is ... It's still too friggin complicated for fried admins to get setup right ... So they just declaw the thing and move on.



















) get carried away, and I hope we can all end up getting along even if we're radically opposed. We obviously have some issues we're pretty darn passionate about, but even if you're a flaming religious person and I'm a flaming atheist, I'm sure there's some thing we don't disagree about.
- carpe noctem
. (Dismounts soapbox, stores it for further usages, retreats into the crowd.)