The human-readable part of the email address (the "friendly name") can contain any character, while the internal or "actual" address is very limited.
A posting to the unicode list a while ago has the following header lines (among others):
Subject: All-kana documents
Date: Mon, 04 Mar 2002 18:47:44 -0500
This switches the encoding of the friendly name to ISO-2022-JP and base64-encodes ("...?B?...") the name.
The same is possible for many other email headers.
See RFC standards.
This archive was generated by hypermail 2.1.2 : Tue May 28 2002 - 16:24:47 EDT