Let me polish this turd a little further eh...
The interception concept is only valid with the private key, you're right, that invites malware and the likes so a good reason to not use that approach but a pending 2of3 escrow however shouldn't suffer from this issue since it wouldn't benefit the scammers.
Also I'm not saying all email must have this 'feature', just emails from external domains can optionally attach it to get put in your 'verified it's not spam' folder. It's not gambling as you would only send these emails to people you want to read it and believe it strongly enough that they'll see why you emailed them.
If you're foolish enough to email someone with an unsolicited sales mail or speak in a tone that pisses them off they'll donate your 'postage fee' away which seems a win/win for the good guys.
Things you should consider first, how are you going to get this done?
Why do you think people needs this then the spam blockers which is actually paid to get rid of spam mails permanently
Why do money have to be involved to (gamble) coz its gambling like a bet, i put bitcoin and you put too let get this going to see who loses his.
Well, think about the rest.. Gotta do something..