Hi, Currently there is no reliable way of pinpointing the origin of an email. This and the spam that it promotes have rendered many email addresses unusable. Why wouldn't this work: The major email software producers would agree to modify their codes such that any email received will automatically request a confirmation from the originating machine referenced in the "from" email address. In other words it would reply an email requesting confirmation from the email client. By default any unconfirmed emails will be discarded unless the user decides otherwise. This would bypass the SMTP servers' unfulfilled role of verifying the originating account.
I think your method would be unwieldy. If nothing else, every single e-mail you ever send would have to be stored, forever (for comparison purposes), on your server. How much space do you reckon that might eat up?
Any anti-spam measure that requires everyone to change servers and clients is destined to fail.
Second problem: I get 1000 clients each to send 1000 emails to different people all claiming to be from you. Your server now gets hit by 1000*1000=1,000,000 requests for confirmation. In other words, you would be giving the world a superb DDOS tool.
It is all client side so no server modification would be required. It would involve some extra overhead but if it can help reduce spam, the bandwidth benefit would outweigh the extra overhead. The timing point is a valid one. The email won't clear until 2 more receipts are sent and received.:( Back to the drawing board.