Editing Textual Editing Textual Inserting Messages
It is often useful to insert a snippet of text from a letter that
someone mailed to provide some context for your reply. The command
C-c C-y (
mh-yank-cur-msg) does this by yanking a portion of
text from the message to which you're replying and inserting `
before each line.
You can control how much text is included when you run this command. If you run this command right away, without entering the buffer containing the message to you, this command will yank the entire message, as is, into your reply.  If you enter the buffer containing the message sent to you and move the cursor to a certain point and return to your reply and run C-c C-y, then the text yanked will range from that point to the end of the message. Finally, the most common action you'll perform is to enter the message sent to you, move the cursor to the beginning of a paragraph or phrase, set the mark with C-SPC or C-@, and move the cursor to the end of the paragraph or phrase. The cursor position is called the point, and the space between the mark and point is called the region. Having done that, C-c C-y will insert the region you selected.
 If you'd rather have the header cleaned up, use C-u r instead of r when replying (see Replying).Editing Textual Editing Textual Inserting Messages