How to add your part to the received email and redirect to a new address?

Question

Python

Good afternoon.
Tell. There is a task.
1. Get a mail message.
2. Get the required fields "From", "To", "Subject"
3. Add your data to the beginning and end of the received message. Something like "Message received at [email protected] and contains the right word"
4. Send to the right mail.
All this was done.

imap_descprt = get_connect_to_server_IMAP("[email protected]", global_check_email_login, global_check_email_password, global_IMAP_port)

imap_descprt.select('INBOX')
status, res = imap_descprt.uid("search", None, "(UNSEEN)")
res = b''.join(res).decode("UTF-8")

if res and status == "OK":
  body = str()

  for res_now in res.split(" "):
    email_from_ = str()
    result, data = imap_descprt.uid("fetch",res_now,"(RFC822)")

    full_mail = email.message_from_bytes(data[0][1], policy=policy.default)

    if full_mail.is_multipart():
      for part in full_mail.get_payload():
        ctype = str()
        charset_type = str(part.get_content_charset())
        ctype = part.get_content_type()
        if ctype == "text/plain" or ctype == "text/html":
          body += part.get_payload(decode = True).decode(charset_type)  # decode
          #body += part.get_payload(decode = False) # No decode
    else:
      charset_type = str(full_mail.get_content_charset())
      body = full_mail.get_payload(decode = True).decode(charset_type)	# decode
      #body = full_mail.get_payload(decode = False)	# No decode

Everything works if the letter itself is simple (that is, all the points are done), the body contains the necessary text, but if it is a beautifully designed html page, then it's tin, waste, Igail. Everything is crooked, terrible.
I tried to add both decoded and not decoded text of the letter in the body and then

msg = MIMEMultipart("mixed")
msg['To'] = email_from
msg['From'] = sender_email
msg['Subject'] = Header(str("REsend: " + email_subject), 'UTF-8')

msg.attach( MIMEText(body_to_send_part_1, "html", _charset='UTF-8') )
msg.attach( MIMEText(body, "plain", _charset='UTF-8') )

It is clear that MIMEText is not suitable for a non-decoded message, and I could not find what is suitable. In general, there are a little more examples of sending letters than necessary, but it is precisely working with data from a letter, unfortunately you have to read the stackoverflow and github
news. Please kick at the necessary examples.
Thank you.

Answer 1

Thank you all =)
In general, the fool himself. No need to parse the message body.
We simply shove it into a new message after the data we need.

msg = MIMEMultipart("mixed")
msg['To'] = email_from
msg['From'] = sender_email
msg['Subject'] = Header(str("REsend: " + email_subject), 'UTF-8')

msg.attach( MIMEText(body_to_send_part_1, "html", _charset='UTF-8') )
msg.attach(full_mail)

Next at the end just send as
The new letter leaves in the same form as it came.