How can security teams detect Open Relay?

What an open relay actually is

An open relay is a mail server that will forward email between outside senders and outside recipients without proper authorization. The practical question is not whether the server can send mail, but whether it accepts and relays mail from unauthorised sources when it should only relay for approved users, trusted hosts, or authenticated sessions.

That distinction matters because a server can appear functional while still being dangerously permissive. A healthy relay policy is narrow, explicit, and testable: the server should reject unauthorised relay attempts, even if it still accepts inbound mail for legitimate local delivery.

For teams building out email security controls, the Email Security Guide is a useful broader reference because it covers relay policy, MX routing, and the surrounding safeguards that determine whether mail flow is actually constrained.

How to test whether a server is relaying mail too broadly

The most direct detection method is controlled outside-in testing. Send a test message from an external source to an external recipient through the target server and observe whether the server accepts and forwards it. If the server relays mail for unauthorised combinations of sender and recipient, that is the core signal of an open relay.

Teams should validate several paths, not just one. Test from an external network, test against different sender and recipient combinations, and compare behaviour for authenticated versus unauthenticated sessions. A server that rejects one case but forwards another may still be misconfigured, even if it is not fully open in the broadest sense.

Review relay rules at the same time. Mail gateways, application relays, smart hosts, and cloud email services often have separate policy layers, so the visible SMTP behaviour may not match the administrator’s intent unless you inspect the actual allowlists, connector rules, and transport restrictions in force.

In environments that use layered email controls, the Email Security Guide also helps teams distinguish intended relay paths from accidental ones, which is important when a relay is only exposed through a narrow connector or hybrid mail flow.

Signals that suggest abuse, not just misconfiguration

Detection is stronger when you pair active testing with operational telemetry. Unexpected outbound spam, sudden mail volume spikes, bounce storms, reputation warnings, and abuse complaints are all signs that a relay may be forwarding traffic it should reject. Those symptoms often appear before an administrator notices a policy mistake.

Teams should also look for repeated relay attempts from unfamiliar IP ranges or user agents, especially when the server logs show acceptance followed by delivery to external domains. That pattern can indicate probing by spammers or abuse automation looking for mail systems that will relay without proper controls.

If the server is part of a broader email platform, review whether the issue is isolated to one connector, one domain, or one authentication path. A partial relay exposure can still be exploited at scale if the server forwards enough messages to support spam, phishing, or backscatter abuse.

Risk and Threat Considerations

An open relay is risky because it turns your mail infrastructure into an abuse platform. Even a small misconfiguration can expose the organisation to spam relaying, reputation damage, blacklisting, and downstream trust loss with recipients and mail providers.

Failure mechanism: The server accepts mail from an unauthorised external sender and forwards it to an external recipient, often because relay restrictions, connector rules, or authentication checks are missing or too broad. Attackers and spam operations look for exactly this behaviour because it gives them a way to move mail through a trusted system.

Impact: The organisation can inherit the sending reputation damage, abuse complaints, and operational load from traffic it did not intend to originate. In severe cases, the relay becomes part of a larger phishing or spam campaign, which increases the chance of blocking, incident response activity, and emergency mail-flow changes.

Practitioner Guidance

What to verify: Confirm the server’s actual relay decision path, not just its documented policy. The key question is whether unauthorised external senders can cause delivery to external recipients through any accepted route, including fallback connectors and hybrid mail paths.

Decision rule: If a controlled outside test succeeds where it should fail, treat the issue as an active mail security defect and prioritise containment before tuning logs or inbox filters. If only one path is exposed, narrow that path first, then re-test from outside to prove the fix.

What good looks like: Legitimate users and trusted hosts can send through approved relay paths, while unauthorised external senders are consistently rejected. The observable result should be repeated failure for unauthorised relay attempts, with no unexpected outbound spam or abuse complaints afterward.

Practitioner takeaway: Open relay detection is about proving negative permission, not just seeing mail flow work. If you cannot demonstrate that unauthorised external relaying fails consistently, the server should be treated as exposed until the policy is corrected and re-tested.

Related resources from NHI Mgmt Group

• How do security teams detect ORB-style relay infrastructure in logs?
• How should security teams detect malicious open source packages when attackers use aliases and code obfuscation?
• How do security teams detect NTLM relay activity before it leads to privilege escalation?
• How should security teams detect malicious open-source packages at scale without relying on slow manual review?