If it's being sent to a gmail address, why not allow the user to undo it as long as the receiver has not looked at it yet?
That said, the "panic button" undo feature seems like a great idea. But 5 seconds is way too short. (I'm envisioning my Dad using this feature in this case, not a techie.) 30 seconds seems perfect -- it's probably not a big deal that your message gets delayed by 30 seconds, and it gives you some 'think time' to remember a detail you forgot to add or fix.
If you disagree, you need to post a comment, not click the downmod button. HN is not Digg.
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.
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.
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.
I doubt it.
Stop doubting :)
I think that feature would be abused.
Currently, my mailbox is so full of junk that if someone wants to send me less mail, I'm extasic.
Really, crying wolf about privacy is one thing, but give me a realistic example.