From: Alice Example <alice@example.com>
To: Bob Example <bob@example.net>
Subject: Nested probe
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="BST-OUTER"

--BST-OUTER
Content-Type: multipart/alternative; boundary="BST-INNER"
X-Part-Index: 1

--BST-INNER
Content-Type: text/plain; charset=us-ascii
X-Part-Index: 1.1

alternative plain
--BST-INNER
Content-Type: text/html; charset=us-ascii
X-Part-Index: 1.2

<p>alternative html</p>
--BST-INNER--
--BST-OUTER
Content-Type: text/plain; charset=us-ascii
X-Part-Index: 2

outer trailing part
--BST-OUTER--