From: Some One <some.one@example.org>
To: Someone Else <someone.else@example.org>
Subject: =?MacChineseTrad?B?T2zpcg==?=
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=zxnrbl

This is the preamble.
--zxnrbl
Content-Type: multipart/alternative; boundary=xyzzy

This is the nested preamble.
--xyzzy
Content-Type: text/plain

Hello!
--xyzzy
Content-Type: text/html

<p>Hello!</p>
--xyzzy--
This is the nested epilogue.
--zxnrbl
Content-Type: text/plain
Content-Disposition: attachment; filename="attachment.txt"

This is the attachment.
--zxnrbl--
This is the epilogue.