rfc2047-header-encoding-alist

rfc2047-header-encoding-alist is a variable defined in `rfc2047.el'.
Its value is

(("Newsgroups")
 ("Followup-To")
 ("Message-ID")
 ("\\(Resent-\\)?\\(From\\|Cc\\|To\\|Bcc\\|\\(In-\\)?Reply-To\\|Sender\\|Mail-Followup-To\\|Mail-Copies-To\\|Approved\\)" . address-mime)
 (t . mime))

Documentation:
*Header/encoding method alist.
The list is traversed sequentially. The keys can either be
header regexps or t.

The values can be:

1) nil, in which case no encoding is done;
2) `mime', in which case the header will be encoded according to RFC2047;
3) `address-mime', like `mime', but takes account of the rules for address
fields (where quoted strings and comments must be treated separately);
4) a charset, in which case it will be encoded as that charset;
5) `default', in which case the field will be encoded as the rest
of the article.