Yank the selected message from the mail reader, as a citation
(message-yank-original).
Fill each paragraph cited from another message
(message-fill-yanked-message).
You can use the command C-c C-y (message-yank-original)
to cite a message that you are replying to. This inserts the
text of that message into the mail buffer. This command works only if
the mail buffer is invoked from a mail reader running in Emacs, such
as Rmail.
By default, Emacs inserts the string ‘>’ in front of each line
of the cited text; this prefix string is specified by the variable
message-yank-prefix. If you call message-yank-original
with a prefix argument, the citation prefix is not inserted.
After using C-c C-y, you can type C-c C-q
(message-fill-yanked-message) to fill the paragraphs of the
cited message. One use of C-c C-q fills all such paragraphs,
each one individually. To fill a single paragraph of the quoted
message, use M-q. If filling does not automatically handle the
type of citation prefix you use, try setting the fill prefix
explicitly. See Filling Text.
You can customize mail citation through the hook
mail-citation-hook. For example, you can use the Supercite
package, which provides more flexible citation
(see Introduction in Supercite).