People use different charsets, and we have MIME to let us know what
charsets they use. Or rather, we wish we had. Many people use
newsreaders and mailers that do not understand or use MIME, and
just send out messages without saying what character sets they use. To
help a bit with this, some local news hierarchies have policies that say
what character set is the default. For instance, the ‘fj’
hierarchy uses iso-2022-jp.
This knowledge is encoded in the gnus-group-charset-alist
variable, which is an alist of regexps (use the first item to match full
group names) and default charsets to be used when reading these groups.
In addition, some people do use soi-disant MIME-aware agents that
aren’t. These blithely mark messages as being in iso-8859-1
even if they really are in koi-8. To help here, the
gnus-newsgroup-ignored-charsets variable can be used. The
charsets that are listed here will be ignored. The variable can be
set on a group-by-group basis using the group parameters (see Group Parameters). The default value is (unknown-8bit x-unknown),
which includes values some agents insist on having in there.
When posting, gnus-group-posting-charset-alist is used to
determine which charsets should not be encoded using the MIME
encodings. For instance, some hierarchies discourage using
quoted-printable header encoding.
This variable is an alist of regexps and permitted unencoded charsets
for posting. Each element of the alist has the form (test
header body-list), where:
is either a regular expression matching the newsgroup header or a variable to query,
is the charset which may be left unencoded in the header (nil
means encode all charsets),
is a list of charsets which may be encoded using 8bit content-transfer
encoding in the body, or one of the special values nil (always
encode using quoted-printable) or t (always use 8bit).
See Encoding Customization in The Emacs MIME Manual, for additional variables that control which MIME charsets are used when sending messages.
Other charset tricks that may be useful, although not Gnus-specific:
If there are several MIME charsets that encode the same Emacs charset, you can choose what charset to use by saying the following:
(put-charset-property 'cyrillic-iso8859-5
'preferred-coding-system 'koi8-r)
This means that Russian will be encoded using koi8-r instead of
the default iso-8859-5 MIME charset.
If you want to read messages in koi8-u, you can cheat and say
(define-coding-system-alias 'koi8-u 'koi8-r)
This will almost do the right thing.
And finally, to read charsets like windows-1251, you can say
something like
(codepage-setup 1251) (define-coding-system-alias 'windows-1251 'cp1251)