Users should be able to append notes to a received message, and ensure that the annotation will be displayed so that it will be distinct from the message itself.
Additional Information:
Users should not be allowed to make changes in received messages. Any such changes would simply provide too much chance for resulting confusion. However, users should be able to append, file, and display their own comments about received messages in some distinctively separate form. If changes are desired in a message itself, then its recipient might make a copy of that message (with appropriate change of its header information) and then edit the copy.