Variable: rmail-secondary-file-regexp
rmail-secondary-file-regexp is a customizable variable defined in
rmail.el.gz.
Value
"\\.xmail\\'"
Documentation
Regexp for which files are secondary Rmail files.
Probably introduced at or before Emacs version 19.23.
Source Code
;; Defined in /usr/src/emacs/lisp/mail/rmail.el.gz
;;;###autoload
(defcustom rmail-secondary-file-regexp "\\.xmail\\'"
"Regexp for which files are secondary Rmail files."
:type 'regexp
:group 'rmail-files)