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.
