This post_process plugin for rss2email/rss2email addresses #101 in a pluggable way.

multipart.py

'''
Converts html messages in multipart messages with both text/html and text/plain parts

  post-process = rss2email.post_process.multipart process


'''

from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText

import html2text

def add_plain(guid: str, message, html: str):
    headers = message.items()
    msg = MIMEMultipart('related')
    for name, value in headers:
        if name.lower().startswith('content-'):
            continue
        msg[str(name)] = value


    alternative = MIMEMultipart('alternative')
    msg.attach(alternative)

    html_part = MIMEText(html, _subtype='html')
    alternative.attach(html_part)

    text_content = html2text.html2text(html=html, baseurl=guid)
    text_part = MIMEText(text_content)
    alternative.attach(text_part)
    return msg

def process(feed, parsed, entry, guid, message):
    if message.get_content_type() == 'text/html':
        m = add_plain(guid, message, message.get_payload())
        return m
    if message.is_multipart():
        # we could support multipart messages, but let's postpone it
        return message
    return message

This requires that this branch is merged upstream: https://github.com/boyska/rss2email/tree/improve-post-process

Otherwise, post-process plugins cannot make so "radical" modifications of the Message