Here's an attack: use your victim's public key to encrypt a pile of important messages addressed to someone else. That poor third party will be unable to open them with their own private key, so they'll have to ask for the key from your victim. Ensure beforehand that the third man is on a different continent than your victim, and just watch your target's email for that message that says "Here's my private key Mr President." Swipe the key, and she's all yours.