After doing a lot of digging around, I started looking in to the python side of the message handling, and noticed that a python object is actually constructing the message to be sent for base64 encoding into the gmail-api message object constructor.

See line 63 from above: message = MIMEText(message_text)

The one trick that finally worked for me, after all the attempts to modify the header values and payload dict (which is a member of the message object), was to set (line 63):

• message = MIMEText(message_text, 'html') <-- add the 'html' as the second parameter of the MIMEText object constructor

The default code supplied by Google for their gmail API only tells you how to send plain text emails, but they hide how they're doing that.ala...message = MIMEText(message_text)

I had to look up the python class email.mime.text.MIMEText object. That's where you'll see this definition of the constructor for the MIMEText object:

• class email.mime.text.MIMEText(_text[, _subtype[, _charset]])We want to explicitly pass it a value to the _subtype. In this case, we want to pass: 'html' as the _subtype.

Now, you won't have anymore unexpected word wrapping applied to your messages by Google, or the Python mime.text.MIMEText object

The Fixed Code

def create_message(sender, to, cc, subject, message_text):    """Create a message for an email.    Args:    sender: Email address of the sender.    to: Email address of the receiver.    subject: The subject of the email message.    message_text: The text of the email message.    Returns:    An object containing a base64url encoded email object.    """    print(sender + ', ' + to + ', ' + subject + ', ' + message_text)    message = MIMEText(message_text,'html')    message['to'] = to    message['from'] = sender    message['subject'] = subject    message['cc'] = cc    pprint(message)    return {'raw': base64.urlsafe_b64encode(message.as_string())}

Try this:

    def CreateMessage(emailSubject, emailTo, emailFrom, message_body, emailCc, html_content=None):        try:            message = MIMEMultipart('alternative')            message['to'] = emailTo            message['from'] = emailFrom            message['subject'] = emailSubject            message['Cc'] = emailCc            body_mime = MIMEText(message_body, 'plain')            message.attach(body_mime)            if html_content:                html_mime = MIMEText(html_content, 'html')                message.attach(html_mime)            return {                'raw': base64.urlsafe_b64encode(                    bytes(                        message.as_string(),                        "utf-8")).decode("utf-8")}        except Exception as e:            print('Error in CreateMessage()', e)            return '400'