How to Set Up SMTP Relay Without Authentication

Published November 3, 2024 · Updated July 14, 2026 · Guides & Tutorials

Sometimes a device or legacy app cannot handle a username and password — a printer, a scanner, an old line-of-business tool. For those cases you can set up an SMTP relay without authentication, using the sender's IP address as the trust signal instead. It works, but it must be locked down carefully or it becomes an open relay that spammers will find within hours.

How unauthenticated (IP-based) relay works

Instead of a login, the relay trusts specific IP addresses. You tell the server "accept mail only from these known internal IPs," and it relays their messages without asking for credentials. Perfect for devices that cannot authenticate — as long as the allowed list is tight.

How to set it up safely

On your relay, enable relaying and add only your specific device IPs to the allowed list.
Never allow relay from 0.0.0.0 or "any" — that is the definition of an open relay.
Point your device at the relay's IP and port (often 25 internally).
Keep the relay on your internal network, not exposed to the public internet.
Ensure the mail leaving the relay still has SPF/DKIM/DMARC for external delivery.

Critical warning: An open relay — one that accepts mail from any IP without auth — will be abused by spammers, get your IP blacklisted, and can take your whole domain's email down. Restrict by IP, always.

Safer than an open relay

A secure dedicated SMTP for your devices

IP-authorised sending, a dedicated IP, and automatic SPF/DKIM/DMARC — relay from any device without the risk of running an open relay. From $39.99/mo.

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Frequently asked questions

How do I set up SMTP relay without a password?

Use IP-based authorisation: configure the relay to accept mail only from specific trusted device IPs. Never allow relay from any address, or you create an open relay.

Is unauthenticated SMTP relay safe?

Only when strictly limited to known internal IPs and kept off the public internet. An open, unrestricted relay is quickly abused and blacklisted.

What's the alternative for devices that can't authenticate?

A dedicated SMTP that supports IP-authorised sending gives those devices a safe, reliable relay with a dedicated IP and proper authentication downstream.