I think there must be some mental process associated with the 'sent' or 'submit' button. It's as though the brain gets new information once those buttons are pressed that was previously unavailable prior to those buttons being pressed. Maybe it's called hindsight, or some type of reflective process.
Does the unconscious mind adopt 'regret' only at specific times, that is, after making a choice?
As an idea to improve emailing, maybe an email client that renames the send button draft 1 then draft 2 then draft 3 before ever showing 'send'. Also, there would be a time delay buffer to stop one's overriding of that 'delayed sending' feature through pressing the button 3 times rapidly in succession. The idea could even be implemented as a Thunderbird addon if one was serious about it, or a greasemonkey script.
Also, I like Digg's submission process of giving 90 seconds to change a posting before it is made permanent, that is, after pressing the 'post' button - it kind of adds a type of pressure which some people need to improve their output given the feature allows textual changes to be made in that time.
Ultimately, the need to proof read and reflect on any written communication multiple times without others' feedback can be a test of one's patience and tenacity. Regret only happens after commitment, so who can ever be certain about a choice...