GMail has been supporting this for some time and recently Outlook.com also implemented a setting available at the mailbox level where every user can configure an "Let me cancel an email I`ve sent" and a time-frame for this (5, 10, 15 or 30 seconds). As far as I can see, Outlook.com`s implementation depends on the browser session, so if you close the browser tab and re-open it, the message is not sent at all.
It would be a great feature for SmarterMail to have something like this, so maybe it can be implemented in the future? Also, it would be great if it would be designed in some other manner so as to not depend just on the WebMail interface, but to work regardless of the mail client used. However, it would be great if it could be configured both at the domain level and/or at the user level.
Maybe it could be a setting that delays sending the mail (submitted to the server either via WebMail or any other client) in the spool for X ammount of time (preferably configurable) and with a button or some other visual cue shown in WebMail that you have X seconds to cancel a sent mail (maybe if the user goes to sent mails, he would have a count-down when he clicks on the sent message with a button to cancel it).
Hope this idea is good and it can be implemented in the future.