Subject: Forwarded thread
From: Alice <alice@example.com>
To: Bob <bob@example.com>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=mix

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

See forwarded message
--mix
Content-Type: message/rfc822; name=forwarded.eml
Content-Disposition: attachment; filename=forwarded.eml
Content-Transfer-Encoding: 7bit

Subject: Inner message
From: Carol <carol@example.com>
To: Bob <bob@example.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8

inner body
--mix--