What Is an Open Relay?

What is an Open Relay?

An open relay is an email server configured to accept and forward messages from any sender to any recipient, even if neither belongs to the server’s own domain.

In the early days of the internet, open relays were common, as mail servers were designed to freely pass messages along. Today, however, this configuration is a major security vulnerability. Spammers and attackers exploit open relays to send large volumes of unsolicited or malicious email, often masking the true origin of their campaigns.

As a result, most modern mail servers are configured as closed relays, meaning they only accept outgoing messages from authorized users or domains.

How Open Relays Work

An open relay allows anyone on the internet to use the server as a middleman for email delivery. For example:

1. A spammer connects to the relay and submits a message “from” fakebank.com to victim@example.com.
2. The open relay, not checking sender authorization, accepts the message.
3. The relay then forwards it to the recipient’s mail server, making it appear as though it came from a legitimate source.

Because the server’s IP address appears in the delivery chain, the open relay’s domain or IP often ends up blacklisted, damaging the sender’s reputation and preventing legitimate mail from being delivered.

Open relays are also used to:

• Obscure the attacker’s real location
• Launch spam or phishing campaigns
• Send malware or fraudulent content
• Distribute spoofed emails that bypass basic filtering

The Risks of an Open Relay

Operating an open relay exposes your domain and network to significant risk. Consequences include:

• Reputation damage: Your IP address or domain may be blacklisted by major providers like Google and Microsoft.
• Deliverability loss: Even legitimate emails from your organization can be blocked or sent to spam.
• Resource abuse: Attackers can overload your mail server, leading to performance issues or service outages.
• Legal exposure: Some jurisdictions impose penalties for enabling or failing to secure open relays.

Administrators can test for open relay vulnerabilities using tools like telnet, nmap, or dedicated online relay checkers to ensure mail servers reject unauthorized senders.

Open Relays and DMARCeye

While DMARCeye focuses on authentication visibility (SPF, DKIM, and DMARC), open relay issues often appear indirectly through anomalous sending activity in DMARC reports.

If your domain’s IP addresses show unusually high volumes of unauthenticated or failing traffic, this may signal that:

• A mail server in your infrastructure is misconfigured as an open relay.
• Unauthorized systems are sending on your behalf.

DMARCeye helps uncover these patterns by visualizing where messages originate and how they perform in authentication checks. By correlating this data, you can quickly spot and shut down potential relay misuse before it harms deliverability or trust.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.