Text inserted at end of mail buffer when a message is initialized.
If t, it means to insert the contents of the file `mail-signature-file'.
If a string, that string is inserted.
(To make a proper signature, the string should begin with \n\n-- \n,
which is the standard way to delimit a signature in a message.)
Otherwise, it should be an expression; it is evaluated
and should insert whatever you want to insert.