Proof of trust - Anti-Spam mechanism

Here is a very rough idea of an anti spam mechanism useful for a crypto mail system:
If you send a mail to someone you need to spend x btc to a collateral address which gets refunded to yourself when the receiver confirms that your message was not spam. That way communication between friends or information which has a value to the receiver stays free and unwanted mails get penalized.
Best would be that the receiver could define individually the amount needed to send a msg to him (people have different tresholds what means spam to them). The money frozen from spam could be forwarded to the receiver himself as compensation for reading the spam (unpayed work) or to the underlaying system (development, network, btc miners,...). Additionally there could be a mechanism to add trusted senders to whitelists so they dont need to pay the collateral anymore...

I have not thought more into implentation scenarios but I assume it should be possible either on top or with btc or as a specialized altcoin project.
A simple starting point could be to use a 2 of 2 multi sig tx (sender and receiver need to sign tx so it can pay out). The sender needs to pay in to that tx and the tx output is his own address. If the mail receiver sign that tx the btc amount will go back to the sender, otherwise its lost to him. That will be for sure not enough (creates burned money...) just a rough idea of the direction....

A specialized system would be probably much more flexible and maybe it could be used to other use cases as well: eg. against ddos attacks, forum postings (you need to pay and get a return if people read/like your posts...),....
Basically all use cases where a misuse happens and its currently free or nearly free to misuse. If you need to risk a certain fund and just get it back if the receiver(s) like your contribution, you could establish a effective filtering ans even build a content conpensation scheme out of it.

I am not sure if something similar already exists and if it is really technically feasible. Just wanted to share the idea....