That's a stupid idea. This isn't a turn based game. What the hell would once a month per recipient do for you? It wouldn't be a useful tool if it didn't provide value to the user at all times. What happens if I screw up much more than one time? Great, I was saved once, but that really didn't do much.
And as for the original idea, it's stupid too. When I browse my inbox, I know exactly what's there, opened and unopened. Once it's sent to me, it's mine. I don't want to see messages appearing/disappearing from my inbox if it's not me doing the manipulating.
I'm all for brainstorming and developing new ideas, but come on, give these ideas some thought.
This isn't a legitimate suggestion. It makes email a gimmick.
I can suggest something as ridiculous by saying, well, if the other user hasn't logged into their account since receiving my message, I should be able to log into their account and delete my message.