On Thu, 12 Jan 2017 00:44:28 +0800, 積丹尼 Dan Jacobson wrote:
> A mail that contains this header

> 	Content-Transfer-Encoding: 8bit;

> causes Gnus to issue the following warning:

> 	Unknown encoding 8bit;; defaulting to 8bit

> This means that Gnus doesn't know the encoding method named:

> 8bit;

> Note that this is not "8bit", but "8bit;".

I found it in a mail that Dan Jacobson sent to me as an example.

> This is due to a bug of a certain mail software.  Gnus does no bad.
> But it is easy and harmless to make Gnus fix it, so, I think we had
> better do something like the above workaround if it generally happens.

A partial[1] patch is below.  `ietf-drums-strip' is an aliasee
of `mail-header-strip' that the following modules use in Emacs:

lisp/gnus/gnus-art.el
lisp/gnus/gnus-sum.el
lisp/gnus/mm-decode.el
lisp/gnus/nndoc.el
lisp/mh-e/mh-mime.el

The purpose of it is all to clean up a string value of
the Content-Transfer-Encoding header of a mail.  So, we can make
`ietf-drums-strip' remove some garbage, such as ";", in addition
to comments and whitespace.  However, other programs outside of
Emacs might be using it for different purposes.  This is why I
added the new function `mail-header-strip-cte' that is assumed
to be exclusively used for cleaning up C-T-E string.

[1] When I install the patch, I will replace `mail-header-strip'
in the modules listed above with `mail-header-strip-cte' as well.

Regards,