xxswatelitexx: Spam really wouldn't work - because you would need to have the users public key first to send the mail and you can only retrieve the public key by requesting it from said user.
So unless you willingly give your public key to a spam account - that shouldn't be an issue.
rtcvb32: Requesting a key may not be a problem, it may come as an attachment since the public key is public. But signing isn't quite the same as just encrypting. Spam is more interested in getting sheer numbers to an unsuspecting populous. Digital signing just ensures it came from said person, as long as the keyID matches. Although if a spammer is known to use public key xxx, then all keys signed with key xxx are automatically filtered out.
Then there's also with known public keys, Yahoo can detect it in the email, and decrypt the message and have a
SIGNED button in green as an indicator making it seamless, as long as the key checks out.
You seem to be mixing a few different concepts.
An authenticated document is not the same as an encrypted document.,
When it comes to Asynchronous encryption.
Every private key has a child key known as a public key.
That public key is unique to every individual.
The public key can only encrypt documents in every specific way that would allow only that specific private key to open.
The only way to view that document is using a specific private key to unencrypt it.
What you are thinking about is document authentication, which uses public certificates authenticated by a 3rd party which has been signed by a private key and authenticated by a public key.