Postfix Mail Relay

How it works?

• Remote Public Postfix Mail Relay Server:
  • Reachble via SMTP at mail.example.com with 10.10.10.10 private IP.
  • Internet (MTA) -> (StartTLS) mail.example.com:25 -> Postfix (Linux/Docker).
  • Postfix -> if relayhost for relay_domains -> 10.10.10.10:1025.
• Local Private Mail Server (Final Destination):
  • SMTP mail.example.lan:25 is the final destination mail server (SMTP).
  • If you prefer SSH Reverse Tunnel:
    • On 10.10.10.10:1025 -> SSH Reverse Tunnel (Linux/Docker) -> Local SMTP.
    • The remote server can connect to the local server via SSH tunnel.
    • DNS resolution for mail.example.lan is needed on the local server.
  • If your prefer Tailscale VPN:
    • On 10.10.10.10:1025 -> TailScale -> Client TailScale VPN -> Local SMTP.
    • Both the remote and local can see each other via the TailScale VPN.
    • The local server is reachable via the TailScale private IP address.
• Includes Realtime Blackhole List (RBL) rejection with Spamhaus.
• Includes SPF verification and OpenDKIM signing/verification.
• (Optional) Includes SSH reverse tunnel monitor Web endpoint.
• See references for more information.

Configuration (Remote and Local Servers)

Download install the mrelay tool:

apt install make git  # If you don't have it
git clone https://github.com/olafrv/mrelay.git
cd mrelay
# If you don't have it (latest docker official version)
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/olafrv/my_collections/refs/heads/main/scripts/bash/install-docker.sh)"

Environment Variables

Create the .env file VARIABLE=VALUE even if empty VARIABLE= with the variable content:

Variable	Example Value	Description
DOCKER_REGISTRY	docker.io/olafrv	The Docker registry to pull the mrelay image from.
MRELAY_TIMEZONE	Europe/Stockholm	The timezone for the container.
MRELAY_POSTFIX_DOMAIN	example.com	The domain name for the Postfix mail server.
MRELAY_POSTFIX_HOSTNAME	mail.example.com	The hostname (MX) for the Postfix mail server.
MRELAY_POSTFIX_RELAYHOST	[mail.example.lan]:25 or empty	The relay host of the Postfix mail server. Empty disables relay.
MRELAY_POSTFIX_CERTBOT_CF_API_KEY	See References for deetails	The Cloudflare API key for Certbot DNS authentication.
MRELAY_POSTFIX_CERTBOT_CF_DNS_WAIT	30	The wait time in seconds for Certbot DNS authentication.
MRELAY_POSTFIX_CERTBOT_EMAIL	joe@example.com	The email address for Certbot cloudflare notifications.
MRELAY_POSTFIX_CERTBOT_FLAG	e.g. --dry-run or empty	Additional flags for Certbot.
MRELAY_POSTFIX_SPAMHAUS_KEY	See References for details	Spamhaus Data Query Service (DQS) key for RBL rejection.
MRELAY_POSTFIX_DKIM_SELECTOR	e.g. default	DKIM selector for OpenDKIM signing/verification.
MRELAY_TUNNEL_SSH_URL	joe@mail.example.com	The SSH URL for the tunnel from the private local rely server.
MRELAY_TUNNEL_SSH_KEY	../id_rsa	The SSH private key file to connect to the public server
MRELAY_TUNNEL_FORWARD	10.10.10.10:1025:mail.example.lan:25	The port forwarding configuration for the tunnel.

Configuration (Public Remote Server)

SSH Server Configuration

Note: This is only needed if you are not using Tailscale VPN.

In the public remote mail server, add the following settings to the /etc/sshd/sshd_config file:

# Allow to forward ports bound to non-localhost address
GatewayPorts yes
# Close unresponsive client (tunnel) connections after 30 seconds
ClientAliveInterval 10
ClientAliveCountMax 3

Then run the following commands:

systemctl restart ssh       # or service ssh restart (apply previous changes)

Postfix Server Configuration

cd mrelay
make postfix.start          # start the container
make postfix.sh             # enter the shell inside the container
make dns                    # to print DNS records needed for SPF and DKIM
make test                   # works only after starting the tunnel
make postfix.stop           # stop the containers
# make postfix.run          # build and run in foreground (development)

To check the SSL certificates, run the following command:

make postfix.certs.list
make postfix.certs.renew
make postfix.certs.renew.force

Tunnel Monitor Configuration

Note: This is only needed if you are not using Tailscale VPN.

make tunnel.monitor.start   # start the tunnel monitor (TCP open/close)
make tunnel.monitor.stop    # stop the tunnel monitor
make tunnel.sh              # enter the shell inside the container
# make tunnel.monitor.run   # build and run in in foreground (development)

The tunnel monitor can be accessed at https://mail.example.com/index.html.

Its content will have OK or ERROR if the TCP port 10.10.10.10:1025 is open or closed, respectively.

To check the SSL certificates, run the following command:

make tunnel.monitor.certs.check

Configuration (Private Local Server)

Tunnel Client Configuration

Note: This is only needed if you are not using Tailscale VPN.

Then run the following command:

make tunnel.start  # start the container
make tunnel.sh     # enter the shell inside the container
make tunnel.stop   # stop the containers
# make tunnel.run  # build and run in foreground (development)

Monitoring and Alerting

You can use Uptime Kuma for monitoring the tunnel monitor HTTP endpoint and/or the postfix SMTP ports.

Development

To build the Docker images locally, run the following command:

make build   # Build the Docker images locally x86_64 and arm64
make push    # Push the Docker images to the registry

References

Certbot SSL Certificates (and DNS Plugins)

• https://eff-certbot.readthedocs.io/en/latest/using.html#manual
• https://eff-certbot.readthedocs.io/en/latest/using.html#dns-plugins
• https://certbot-dns-cloudflare.readthedocs.io/en/stable/
• https://letsencrypt.org/

OpenSSH Client and Server Configuration

• https://man.openbsd.org/ssh_config
• https://man.openbsd.org/sshd_config

TailScale VPN

• https://tailscale.com/kb/1151/what-is-tailscale
• https://tailscale.com/kb/1031/install-linux

Tackling E-Mail Spoofing and Phishing

• https://blog.cloudflare.com/tackling-email-spoofing/

SPAMHAUS Data Query Service (DQS):

• https://www.spamhaus.org/faq/section/DNSBL%20Usage
• https://www.spamhaus.com/free-trial/sign-up-for-a-free-data-query-service-account/

Mail Server Checks (Online Tools)

• https://mxtoolbox.com/
• https://www.spf-record.com/
• https://dmarc.org/resources/deployment-tools/

Sender Policy Framework (SPF)

• https://www.cloudflare.com/learning/dns/dns-records/dns-spf-record/
• https://datatracker.ietf.org/doc/html/rfc7208
• https://www.spf-record.com/

DomainKeys Identified Mail (DKIM)

• https://www.cloudflare.com/learning/dns/dns-records/dns-dkim-record/
• https://linux.die.net/man/8/opendkim
• https://linux.die.net/man/5/opendkim.conf
• https://datatracker.ietf.org/doc/html/rfc6376

Domain-based Message Authentication, Reporting & Conformance (DMARC)

• https://www.cloudflare.com/learning/dns/dns-records/dns-dkim-record/
• https://dmarc.org/overview/
• https://dmarc.org/resources/specification/