Every few months we get into a discussion about editing messages. Is it OK to edit a message after it has been sent--to correct a typo, to fix a formatting error, to change the content, etc.
My argument usually goes that you should not go back and edit becaue of email subscriptions. Always follow up with a correction/change so that everyone is on the same page, no matter how they follow the conversation (web, email subscription, etc.). This is especially important if the edit changes the context of the message (accidentally sent, "please do install this widget" intead of "please do NOT install this widget").
But what about small typos or formatting issues? Very tempting to go change the original. And probably won't hurt. But could. So, that brings up the request to be able to preview a message post to see formatting, possibly catch a typo or a dropped word, etc.