ispell-message-dictionary-alist

ispell-message-dictionary-alist is a variable defined in `ispell.el'.
Its value is

nil

Documentation:
List used by `ispell-message' to select a new dictionary.
It consists of pairs (REGEXP . DICTIONARY). If REGEXP is found
in the message headers, `ispell-local-dictionary' will be set to
DICTIONARY if `ispell-local-dictionary' is not buffer-local.
E.g. you may use the following value:
'(("^Newsgroups:[ \t]*de\\." . "deutsch8")
("^To:[^\n,]+\\.de[ \t\n,>]" . "deutsch8"))

You can customize this variable.