KMail has a Undo Send feature.
Undo Send: Delays sending a message for 10/20/30/40/50 seconds, allowing time to stop the message from being sent.
Enable Undo Send: Select Settings > Configure KMail > Accounts > Sending > Enable Undo Send (select checkbox, this will activate the drop down menu to select time delay) > Apply > OK.
When Send is selected to send a message, a Undo Send button will appear in the notification area that will stop the message from being sent if selected in the allotted time. The message can be edited and sent again or deleted.
