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

--longrandomstring
Content-Type: text/html

<html><body>This is body of <b>HTML mail</b> with attachment</body></html>
--longrandomstring
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64

b'aWYgeW91IGFyZSByZWFkaW5nIHRoaXMsIGhpIQ=='
--longrandomstring--
