By default, Gmail tells you when your contacts are online anyway... so if you want to see if someone is reading their email, you can just click their name and send them an IM. This doesn't invade their privacy any more than that does.
(BTW, the undo conditional should be "seen", not read. Basically, if the recipient has logged into Gmail since you've sent the message, undo should not work.)
Emails can be forwarded automatically, notification of new emails can be sent to mobile devices, they can be tagged/labeled/moved/starred, add-ons or third party apps can screen scrape and process that information... all without a message being tagged as "read" and some of these without even being logged in.
If you want to implement a think-it-over delay in your client, go right ahead. If you want to arrange something with your provider (Gmail) to be able to stop delivery (a la UPS) then go right ahead, but you're going to have to be mighty quick.
Once its in my inbox, however, it's mine. Any provider who would do such a thing would certainly loose me as a subscriber. Your best bet is a nicely worded apology letter.
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.
