Header contains information used for transportation, so it should be strongly prohibited to insert improper characters that make transport agents misoperated. With MIME, non-ASCII characters are encoded into transport-safe characters then stored as a field value with the following format.
=?<charset>?<encoding>?<encoded-string>?=
<charset> is identical to the charset parameter of CT: Text/Plain. For <encoding>, `B' or `Q' is used. The former is exactly base64 and the latter is a kind of Quoted-Printable.
For ISO-2022-JP, `B' is encouraged. `Q' is also acceptable, however, few message interfaces support it(of course, Mew does).
For instance, the author's Japanese name in Subject: is encoded as follows:
Subject: =?ISO-2022-JP?B?GyRCOzNLXE9CSScbKEI=?=
It is not parameter values but field values that this format can handle. One of the reasons why this format must not be applied to parameter values is that the "=" keyword conflicts the separator between parameter names and parameter values. To encode non-ASCII characters in a parameter value, another format should be used. Please see the following example to understand the differences:
Content-Disposition: attachment; filename*=iso-2022-jp''%1B%24BF%7CK%5C8l%24N%25U%25%21%25%24%25k%1B%28B