Is there a spell-checker? Perhaps even on-the-fly spell-checking?
You can use ispell.el to spell-check stuff in Emacs. So the first thing to do is to make sure that you’ve got either ispell or aspell installed and in your Path. Then you need ispell.el and for on-the-fly spell-checking flyspell.el. Ispell.el is shipped with Emacs and available through the XEmacs package system, flyspell.el is shipped with Emacs and part of XEmacs text-modes package which is available through the package system, so there should be no need to install them manually.
Ispell.el assumes you use ispell, if you choose aspell say
(setq ispell-program-name "aspell")
in your Emacs configuration file.
If you want your outgoing messages to be spell-checked, say
(add-hook 'message-send-hook 'ispell-message)
In your ~/.gnus.el, if you prefer on-the-fly spell-checking say
(add-hook 'message-mode-hook (lambda () (flyspell-mode 1)))