From: Sender <sender@pm.me>
To: Receiver <receiver@pm.me>
Content-Type: multipart/mixed; boundary=longrandomstring

--longrandomstring

body
--longrandomstring
Content-Type: message/rfc822
Content-Disposition: attachment

From: Sender <sender@pm.me>
To: Receiver <receiver@pm.me>

inner body
--longrandomstring--