How to Windows server smtp relay

Published October 20, 2024 · Updated August 26, 2026 · Guides & Tutorials

If you run Windows Server, you can turn it into an SMTP relay so applications, scanners, and internal systems can send email without each one needing its own mail account. Windows has shipped an SMTP relay feature for years — it is handy for internal routing, though it is not a bulk-marketing platform. Here is how to set it up and where its limits begin.

What the Windows Server SMTP relay does

The built-in SMTP Server feature (part of IIS 6 management tools) accepts mail from your internal devices and forwards it on. It is perfect for letting a multifunction printer email scans, or a legacy app send notifications, through one central point. It relays; it does not give you a dedicated sending IP or marketing-grade deliverability.

How to set up the SMTP relay feature

In Server Manager → Add Roles and Features, install the SMTP Server feature.
Open IIS 6.0 Manager and expand the SMTP Virtual Server.
Under Access → Relay, allow only your trusted internal IP addresses to relay.
Set the smart host or delivery settings to forward mail onward.
Point your apps and devices at the server's IP on port 25 (internal) and test.

Two cautions: lock the relay down to trusted IPs only (an open relay gets abused within hours), and remember that mail leaving your Windows box still needs SPF/DKIM/DMARC and a good sending IP to reach external inboxes.

A note on IIS 6 SMTP's age

The built-in SMTP Server feature is legacy technology, tied to older IIS management tooling that Microsoft has not meaningfully updated in years — it still works for basic internal relay, but do not expect modern authentication conveniences or active development. For anything beyond simple internal forwarding, most admins now reach for a dedicated relay rather than depending on it long-term.

Monitoring the relay queue

The SMTP Virtual Server keeps a local queue directory for mail awaiting delivery — check it periodically (or set up basic monitoring) for messages stuck retrying, which usually signals the downstream smart host is rejecting them, often due to missing authentication on the outbound domain.
Related reading: Linux smtp relay · have 2 domains on one host · free email tools

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Frequently asked questions

Does Windows Server include an SMTP relay?

Yes — the SMTP Server feature, managed through IIS 6.0 tools. It relays mail from internal apps and devices through one central point.

Is the Windows SMTP relay good for bulk email?

No. It is designed for internal routing, not marketing volume or deliverability. Forward its mail to a dedicated SMTP smart host for external and bulk sending.

How do I stop my Windows SMTP server being an open relay?

Under Access → Relay, permit only specific trusted internal IPs. Never allow anonymous relay from any address, or spammers will abuse it and blacklist your IP.