From: someone@example.com
To: test@example.com
Subject: something else
Date: Fri, 01 Jan 2021 00:00:00 +0000
Message-ID: <sa2@example.com>
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="bnd123"

--bnd123
Content-Type: text/plain; charset=UTF-8

Plain part

--bnd123
Content-Type: text/html; charset=UTF-8

<html><body>HTML part</body></html>

--bnd123--
