mailrelay

mailrelay is a simple mail relay that can take unauthenticated SMTP emails (e.g. over port 25) and relay them to authenticated, TLS-enabled SMTP servers. Plus it's easy to configure.

Prebuilt binaries are available here for Linux, MacOS, Windows, OpenBSD.

Use case

Some older appliances such as scanners, multi-function printers, RAID cards or NAS boxes with monitoring, can only send email without any authentication or encryption over port 25. mailrelay can send those emails to your Gmail, Fastmail or other provider.

Run mailrelay on a local PC or server and set your device (e.g. scanner) to send mail to it.

mailrelay is written in Go, and can be compiled for any Go supported platform including Linux, MacOS, Windows.

Encryption

mailrelay uses TLS to connect to your SMTP provider. By default implicit TLS connections are assumed, meaning the connection is established using TLS at the socket level. This is in accordance with RFC 8314 section 3. These connections usually use port 465.

However, some providers do not adhere to this recommendation (I'm looking at you Office365!) and only support the legacy STARTTLS command, which expects a non-encrypted socket connection at first, which is then upgraded to TLS. To enable this, set smtp_starttls to true in your config. These connections usually use port 587.

Testing your configuration

You can send a test email using the -test flag. A email will be sent using the SMTP provider specified in your mailrelay.json configuration.

./mailrelay -config=./mailrelay.json -test -sender=dlauder@warpmail.net -rcpt=ender.wiggin@warpmail.net

Example (Linux)

On local PC (192.168.1.54) create file /etc/mailrelay.json with contents:

/etc/mailrelay.json

{
    "smtp_server":   "smtp.fastmail.com",
    "smtp_port":     465,
    "smtp_starttls": false,
    "smtp_username": "username@fastmail.com",
    "smtp_password": "secretAppPassword",
    "local_listen_ip": "0.0.0.0",
    "local_listen_port": 2525,
    "allowed_hosts": ["*"]
}

Run mailrelay,

./mailrelay

Default location for configuration file is /etc/mailrelay.json but can be changed via --config flag. For example,

mailrelay --config=/home/myname/mailrelay.json

Configure your scanner or other device to send SMTP mail to server 192.168.1.54:2525. Each email will be relayed to smtp.fastmail.com using the credentials above, including any file attachments.

Example 2 (Linux - Systemd service)

Create configuration file as above, and also create,

/etc/systemd/system/mailrelay.service

[Unit]
Description=Mail Relay Service
After=network.target
StartLimitIntervalSec=0

[Service]
Type=simple
Restart=always
RestartSec=5
ExecStart=/usr/local/bin/mailrelay

[Install]
WantedBy=multi-user.target

Copy mailrelay to /usr/local/bin/.

Run,

sudo systemctl start mailrelay
sudo systemctl enable mailrelay

Now mailrelay runs as a service daemon and will automatically start after reboot.

Feedback

Send any questions or comments to wiggin77@warpmail.net