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Proxy to SMTP

SMTP is a protocol used for sending emails. In the simpliest setup, it involves a SMTP client and a SMTP server to send and receive emails.

In a more complicated fashion, it can involves a number of proxies to ensure emails are sent, a number of firewall filters to filter inbound and outbound emails, and a number of exchange servers for setting up mailing lists and groups.

SSH Configuration​

Proxying to send emails requires us to setup a forwarding port to send traffic to.

In the configuration below, we have to

  1. establish a SSH tunnel to the example-network using an SSH key pair (identity file) on port 22.
  2. setup local port forwarding to a local port 5025 where the traffic going to localhost:5025 will go to smtp.example.com on port 25 via the SSH tunnel.
info

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SSH configuration:

Host example-network
  HostName 192.168.0.1
  User someusername
  PasswordAuthentication no
  PubkeyAuthentication yes
  IdentityFile ~/.ssh/example_network_id_ed25519
  IdentitiesOnly yes
  Port 22
  ServerAliveInterval 10
  ServerAliveCountMax 2
  Compression yes
  TCPKeepAlive yes
  GatewayPorts yes
  RemoteForward 5020
  RemoteForward 5025 smtp.example.com:25

Sending Emails Manually​

Test emails can be sent using the smtp protocol using telnet or simple python code without dependencies.

This code below will connect to localhost:5025 using the python smtp library to send an email from email@example.com to email@example.com.

info

This code will only work with unauthenticated smtp / exchange server.

import os
import smtplib
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart

hostname = os.environ.get('host', 'localhost')
port = os.environ.get('port', '5025')
sender_email = os.environ.get('sender_email', 'email@example.com')
receiver_email = os.environ.get('receiver_email', 'email@example.com')

message = MIMEMultipart("alternative")
message["Subject"] = "Welcome Onboard"
message["From"] = sender_email
message["To"] = receiver_email
message["Cc"] = receiver_email

html = """
  <html>
    <body>
      Hello, email from us!
    </body>
  </html>
"""

part1 = MIMEText(html, "plain")
message.attach(part1)

with smtplib.SMTP(hostname, port) as server:
  server.sendmail(
    sender_email, receiver_email, message.as_string()
  )