quick mod of a SES forwarder Lambda function

ses-forwarder.py

"""
This code is a Python 3.6 port of [aws-lambda-ses-forwarder](https://github.com/arithmetric/aws-lambda-ses-forwarder). Follow instructions there for setting up SES and AWS Lambda. It was ported to py2.7 by [skylander86](https://gist.github.com/skylander86/d75bf010c7685bd3f951d2b6c5a32e7b), and then I added the following:
- py3 compatability, obviously.
- MSG_TARGET and MSG_TO_LIST: move the distribution list out of code.
- SUBJECT_PREFIX: add something like `[listname]` to the subject line.
- S3_PREFIX: an optional prefix for the key used to fetch a mail message. Useful if you put your incoming mail in an s3 'directory'.
- Commented out 'from rewriting', instead using 'reply-to' to redirect replies back to the list.
The original was MIT licensed; skylander86's gist doesn't have a license, so it is presumed to still be MIT. This version is copyright 2018 tedder, MIT license.
"""

import email
import json
import logging
import os
import re

import boto3
from botocore.exceptions import ClientError

FORWARD_MAPPING = {
    os.environ.get('MSG_TARGET'): os.environ.get('MSG_TO_LIST'),
}

VERIFIED_FROM_EMAIL = os.environ.get('VERIFIED_FROM_EMAIL', 'noreply@truelabs.io')  # An email that is verified by SES to use as From address.
SUBJECT_PREFIX = os.environ.get('SUBJECT_PREFIX') # label to add to a list, like `[listname]`
SES_INCOMING_BUCKET = os.environ['SES_INCOMING_BUCKET']  # S3 bucket where SES stores incoming emails.
S3_PREFIX = os.environ.get('S3_PREFIX', '') # optional, if messages aren't stored in root
s3 = boto3.client('s3')
ses = boto3.client('ses')

logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)


def lambda_handler(event, context):
    record = event['Records'][0]
    assert record['eventSource'] == 'aws:ses'

    o = s3.get_object(Bucket=SES_INCOMING_BUCKET, Key=S3_PREFIX+record['ses']['mail']['messageId'])
    logger.info("loading Message ID {} from S3".format(record['ses']['mail']['messageId']))
    raw_mail = o['Body'].read()
    #logger.info("body: {}".format(type(raw_mail)))
    #msg = raw_mail
    msg = email.message_from_bytes(raw_mail)
    #logger.info("m: {}".format(msg))

    del msg['DKIM-Signature']
    del msg['Sender']
    del msg['Return-Path']
    del msg['Reply-To']

    logger.info("{} keys: {}".format(record['ses']['mail']['messageId'], msg.keys()))
    logger.info("{} from: {}".format(record['ses']['mail']['messageId'], msg['From']))
    #original_from = msg['From']
    #del msg['From']
    #msg['From'] = re.sub(r'\<.+?\>', '', original_from).strip() + ' <{}>'.format(VERIFIED_FROM_EMAIL)

    original_from = re.findall(r'[\w\.-]+@[\w\.-]+', msg['From'])
    if len(original_from) == 1:
        msg['Reply-To'] = original_from[0]
    else:
        msg['Reply-To'] = VERIFIED_FROM_EMAIL
        
    msg['Return-Path'] = VERIFIED_FROM_EMAIL

    logger.info("{} subject prefix: {}".format(record['ses']['mail']['messageId'], SUBJECT_PREFIX))
    if SUBJECT_PREFIX and SUBJECT_PREFIX.lower() not in msg.get('Subject').lower():
        new_subj = ' '.join([SUBJECT_PREFIX, msg.get('Subject', '')])
        del msg['Subject']
        msg['Subject'] = new_subj
        logger.info("{} new subj: {}".format(record['ses']['mail']['messageId'], msg['Subject']))

    msg_string = msg.as_string()

    for recipient in record['ses']['receipt']['recipients']:
        logger.info("{} recipient: {}".format(record['ses']['mail']['messageId'], recipient))
        forwards = FORWARD_MAPPING.get(recipient, '')
        if not forwards:
            logger.warning('{} Recipent <{}> is not found in forwarding map. Skipping recipient.'.format(record['ses']['mail']['messageId'], recipient))
            continue

        for address in forwards.split(','):
            logger.info("{} addr: {}".format(record['ses']['mail']['messageId'], address))

            try:
                o = ses.send_raw_email(Destinations=[address], RawMessage=dict(Data=msg_string))
                logger.info('{} Forwarded email for <{}> to <{}>. SendRawEmail response={}'.format(record['ses']['mail']['messageId'], recipient, address, json.dumps(o)))
            except ClientError as e: logger.error('{}, Client error while forwarding email for <{}> to <{}>: {}'.format(record['ses']['mail']['messageId'], recipient, address, e))