From: a@example.com
Subject: blah
Cc: test-public@example.com
Message-ID: <multipart-html-sucks@11>
Content-Type: multipart/alternative; boundary="b"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

--b
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: base64

PGh0bWw+PGJvZHk+aGk8L2JvZHk+PC9odG1sPg==

--b
Content-Type: text/plain
Content-Transfer-Encoding: quoted-printable

hi =3D "bye"=

--b--