If you're accidentally sending messages using other keys there are at least three workarounds you can use to stop the message from being sent.
First method: type random characters in the CC field so the "address" won't resolve. This stops the send and brings up the Check names dialog, which allows you to cancel the send.
Second method: enable the option to check spelling before sending then intentionally misspell a word.
The third method will delay all messages by the specified number of minutes. It uses an After sending rule, with the action to Defer delivery by a specified number of minutes. While you can defer delivery for up to 120 minutes, 1 to 2 minutes will give you plenty of time to get the message out of the Outbox.
Using an after sending rule also gives you a little extra time if you often want to edit messages before they are sent and use Send Immediately. If you want some messages to bypass the rule and be sent without delay, add an Exception to the rule.