Posting styles are using in Gnus to set various headers of outgoing messages (mail and news) depending on which group you are in. Read the Gnus manual for more.

Here is an example.

 (setq gnus-posting-styles
	  (signature "\nCoffee should be black as hell, strong as death and sweet as love.\n\t-- Turkish proverb"))
	  (signature "drsrm0.02 t0 ss12 d0 SfDSA Rwo A0 Dr1w1 T*on W3,4 a5 xsmr1 E7 Vr GzP,Fsp,o,h\n"))
	  (signature (concat "\n\""
			     (vfaq-entry) "\"")))
	  ("Followup-To" ""))
	  (signature "\n")
	  (name "Kensanata")
	  (address ""))
	  (signature "Foo, Inc.
 Some Street and Number, Zip Code and City
 Phone Number for the Internet Challenged
	  ("Reply-To" "")
	  ("Cc" (unless (condition-case nil 
			    (symbol-value 'header)
			  ('void-variable nil)) "")))))

Here’s something I just did with posting-styles. Here at work we use the MDaemon mail server. It is configured to allow relaying iff you use a valid “From:” address (note that IP address is not taken into account at all). So, I have to set my “From:” address to be my work address. However, I don’t always want to use my work address. So, I’m able to use posting-styles to handle conditional setting of “Reply-To:”. This basically says to default to using my own mail address if the ‘m’ key is hit from the Group buffer. If I’m replying to an article, then use whatever was in the “To:” field of the article. Also, if I’m sending a message from within my work account (always nnml backend), use my work address. It’s much simpler in elisp than english. --LathI

 (setq gnus-posting-styles
         ("Reply-To" (save-excursion
                       (or (and (get-buffer gnus-article-buffer)
		                (set-buffer gnus-article-buffer)
		                (message-fetch-field "To"))
          ("Reply-To" "mailbox@work"))))