AWS SES邮件转Slack Webhook的Lambda POST请求故障排查求助

我正在搭建「个人邮箱→AWS SES→S3→Lambda→POST请求」的工作流，目标是将AWS SES接收的邮件转发至Slack Webhook，但Lambda函数无法成功向Webhook URL发送POST请求。以下是我的Lambda代码及触发时的CloudWatch日志，恳请帮忙排查问题：

from ast import parse
import boto3
import ConfigParser
import urllib3 
import json
from email.parser import FeedParser
from email.header import decode_header
http = urllib3.PoolManager() 

def lambda_handler(event, context):
  try:
    record = event["Records"][0]
    bucket_region = record["awsRegion"]
    bucket_name =  record["s3"]["bucket"]["name"]
    mail_object_key = record["s3"]["object"]["key"]
  
    s3 = boto3.client('s3', region_name=bucket_region)
    mail_object = s3.get_object(Bucket = bucket_name, Key = mail_object_key)
    mail_body = ''
    try:
      mail_body = mail_object["Body"].read().decode('utf-8')
    except:
      mail_body = mail_object["Body"].read()
  
    parser = FeedParser()
    parser.feed(mail_body)
    parsed_mail = parser.close()
  
    (d_sub, sub_charset) = decode_header(parsed_mail['Subject'])[0]
    subject = d_sub.decode(sub_charset)
    payload = parsed_mail.get_payload(decode=parsed_mail['Content-Transfer-Encoding'])
    body_charset = parsed_mail.get_content_charset()
    body = payload.decode(body_charset)

    url = "MY_SLACK_WEBHOOK_URL"
    msg = {
        "Content": parsed_mail
    }
    encoded_msg = json.dumps(msg).encode('utf-8')
    resp = http.request('POST',url, body=encoded_msg)
    print({
        "message": parsed_mail, 
        "status_code": resp.status, 
        "response": resp.data
    })
  except:
    print('Mail received, but I got some error.')

CloudWatch日志：

START RequestId: 00f3e7db-807e-48e9-a775-6f0117431b83 Version: $LATEST
Mail received, but I got some error.
END RequestId: 00f3e7db-807e-48e9-a775-6f0117431b83
REPORT RequestId: 00f3e7db-807e-48e9-a775-6f0117431b83  Duration: 1933.59 ms    Billed Duration: 1934 ms    Memory Size: 128 MB Max Memory Used: 74 MB  Init Duration: 317.15 ms    

内容的提问来源于stack exchange，提问作者girifir