• Gnus 5.11 Reference Card
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I use Gnus for reading/writing e-mails. To open an Emacs mail buffer automatically, when clicking a HTML mailto link the following shell script might be useful:

#!/bin/sh
# Emacs mailto handler
 
#choose your line below depending on your favorite web browser, used desktop GUI respective XDG settings.
emacsclient -a /usr/bin/emacs -c -e "(browse-url-mail \"$@\")"
#emacsclient -a /usr/bin/emacs -e "(browse-url-mail \"mailto:$@\")"

To open Gnus automatically on C-x m (compose-mail) i guess the following advice from Stack Overflow:

(defadvice gnus-msg-mail (before start-gnus activate)
  (require 'gnus-start)
  (unless (gnus-alive-p)
    (save-window-excursion
      (let ((inhibit-redisplay t))
        (gnus)))))