You most often end up in a Message buffer when responding to some other message of some sort. Message does lots of handling of quoted text, and may remove signatures, reformat the text, or the like—depending on which used settings you're using. Message usually gets things right, but sometimes it stumbles. To help the user unwind these stumblings, Message sets the undo boundary before each major automatic action it takes. If you press the undo key (usually located at C-_) a few times, you will get back the un-edited message you're responding to.