Hi Sven

Maybe this is what you need:

(setq gnus-posting-styles
      '(("emacs-liste"
	 (address "sven.bretfeld@gmx.ch")
	 (eval (setq ispell-local-dictionary "american")))
	("auctex-liste"
	 (address "sven.bretfeld@gmx.ch")
	 (eval (setq ispell-local-dictionary "american")))))

emacs-liste and auctex-liste are names of mailboxes. If I send a
message to one of these groups, my mail-address is automatically
changes to the gmx.ch and the ispell-dictionary switches to "american"
instead of "swiss" which is my default.

Greetings,

Sven