Reply to email using python 3.4

I am trying to reply to an email using Python 3.4. The recipient of the email will be using Outlook (unfortunately) and it is important that Outlook recognizes the reply and displays the thread properly.

The code I currently have is:

def send_mail_multi(headers, text, msgHtml="", orig=None):
    """
    """

    msg = MIMEMultipart('mixed')
    # Create message container - the correct MIME type is multipart/alternative.
    body = MIMEMultipart('alternative')

    for k,v in headers.items():
        if isinstance(v, list):
            v = ', '.join(v)
        msg.add_header(k, v)

    # Attach parts into message container.
    # According to RFC 2046, the last part of a multipart message, in this case
    # the HTML message, is best and preferred.
    body.attach(MIMEText(text, 'plain'))
    if msgHtml != "": body.attach(MIMEText(msgHtml, 'html'))
    msg.attach(body)

    if orig is not None:
        msg.attach(MIMEMessage(get_reply_message(orig)))
        # Fix subject
        msg["Subject"] = "RE: "+orig["Subject"].replace("Re: ", "").replace("RE: ", "")
        msg['In-Reply-To'] = orig["Message-ID"]
        msg['References'] = orig["Message-ID"]+orig["References"].strip()
        msg['Thread-Topic'] = orig["Thread-Topic"]
        msg['Thread-Index'] = orig["Thread-Index"]
    
    send_it(msg['From'], msg['To'], msg)

The function get_reply_message is removing any attachments as in this answer.
send_it function sets the Message-ID header and uses the proper SMTP configuration. Then it calls smtplib.sendmail(fr, to, msg.as_string())
Outlook receives the email but does not recognize/display the thread. However, the thread seems like being an attachment to the message (probably caused by msg.attach(MIMEMessage(...))

Any ideas on how to do this? Have I missed any headers?

Solution

Took me a while but the following seems working:

def send_mail_multi(headers, text, msgHtml="", orig=None):
    """
    """

    msg = MIMEMultipart('mixed')
    # Create message container - the correct MIME type is multipart/alternative.
    body = MIMEMultipart('alternative')

    for k,v in headers.items():
        if isinstance(v, list):
            v = ', '.join(v)
        msg.add_header(k, v)

    # Attach parts into message container.
    # According to RFC 2046, the last part of a multipart message, in this case
    # the HTML message, is best and preferred.
    if orig is not None:
        text, msgHtml2 = append_orig_text(text, msgHtml, orig, False)

        # Fix subject
        msg["Subject"] = "RE: "+orig["Subject"].replace("Re: ", "").replace("RE: ", "")
        msg['In-Reply-To'] = orig["Message-ID"]
        msg['References'] = orig["Message-ID"]#+orig["References"].strip()
        msg['Thread-Topic'] = orig["Thread-Topic"]
        msg['Thread-Index'] = orig["Thread-Index"]

    body.attach(MIMEText(text, 'plain'))
    if msgHtml != "": 
        body.attach(MIMEText(msgHtml2, 'html'))
    msg.attach(body)

    send_it(msg)


def append_orig_text(text, html, orig, google=False):
    """
    Append each part of the orig message into 2 new variables
    (html and text) and return them. Also, remove any 
    attachments. If google=True then the reply will be prefixed
    with ">". The last is not tested with html messages...
    """
    newhtml = ""
    newtext = ""

    for part in orig.walk():
        if (part.get('Content-Disposition')
            and part.get('Content-Disposition').startswith("attachment")):

            part.set_type("text/plain")
            part.set_payload("Attachment removed: %s (%s, %d bytes)"
                        %(part.get_filename(), 
                        part.get_content_type(), 
                        len(part.get_payload(decode=True))))
            del part["Content-Disposition"]
            del part["Content-Transfer-Encoding"]

        if part.get_content_type().startswith("text/plain"):
            newtext += "\n"
            newtext += part.get_payload(decode=False)
            if google:
                newtext = newtext.replace("\n","\n> ")

        elif part.get_content_type().startswith("text/html"):
            newhtml += "\n"
            newhtml += part.get_payload(decode=True).decode("utf-8")
            if google:
                newhtml = newhtml.replace("\n", "\n> ")

    if newhtml == "":
        newhtml = newtext.replace('\n', '<br/>')

    return (text+'\n\n'+newtext, html+'<br/>'+newhtml)

The code needs a little bit tiding up but as is Outlook displays it correctly (with Next/Previous options). There was no need to create From, Send, To, Subject headers by hand, appending the content worked.

Hope this saves someone else's time