Attach region or whole buffer to new mail.

sw-email.el

 (defun mail-current-buffer ()
   (interactive)
   (save-excursion
     (if (region-active-p)
         (kill-ring-save (region-beginning) (region-end))
       (kill-ring-save (point-min) (point-max)))

     (mail)
     (next-line 3)
     (yank)
     (beginning-of-buffer)
     (next-line)
     (end-of-line)))