In 10.2, the process is the same as 10.1, but the menu item has been renamed. To resend a message, highlight the message of interest in your Sent Items folder and then select File -> Open as New Message. You can then edit everything about the message (including the sending account) and hit the Send button. Why this menu option doesn't have a keyboard shortcut (and isn't located in the Message menu) are questions only the Apple Mail engineers can answer!

