+If `system-default', allows the mailer to insert its default From field
+derived from the envelope-from address.
+
+In old versions of Emacs, the `system-default' setting also caused
+Emacs to pass the proper email address from `user-mail-address'
+to the mailer to specify the envelope-from address. But that is now
+controlled by a separate variable, `mail-specify-envelope-from'."