This is a major mode for use with mutt, slrn, or most email and newsreaders that allow you to use an external editor.
Add the following line to the .emacs(.el) file in your home directory:
(load "/your/local/path/to/this/file/post")
Note that you can omit the ".el" from the file name when calling load.
If you want to make it available to all your users, type \C-h v load-path RET, pick an appropriate directory for post.el, and modify
your sitewide default.el to (require 'post).
You may find the latest version of this mode at http://github.com/zedinosaur/post-mode/