Subject: Alternative
From: Alice <alice@example.com>
To: Bob <bob@example.com>
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=alt

--alt
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

hello
--alt
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 7bit

<p>hello</p>
--alt--