Loop Detection (Email)

What Is Loop Detection?

Loop detection is a process used by mail servers to identify and prevent emails from being routed repeatedly between systems without ever reaching a final destination.

A mail loop occurs when two or more servers continue to forward the same message back and forth, often due to misconfigured forwarding rules or automatic responses. Loop detection ensures that messages caught in this cycle are stopped before they consume system resources or cause message floods.

How Does Loop Detection Work?

When an email is sent, each mail server that handles it adds a Received header to the message. These headers record the path the message takes through the network.

Loop detection mechanisms examine these headers for signs of repeated delivery paths or an unusually high number of hops. If a message exceeds a predefined hop count (commonly 25 or 30), or if it repeatedly appears to originate from the same servers, the system flags it as a potential loop.

Once detected, the mail server typically rejects the message or generates a non-delivery report to stop the cycle. Some systems also check for specific X-Loop headers, which are used to mark messages that have already triggered an automated response, such as a vacation reply or mailing list bounce.

Common Causes of Mail Loops

Mail loops can arise from several configuration issues, including:

• Circular forwarding between two or more user accounts
• Misconfigured mailing lists or alias addresses
• Auto-responders replying to one another
• Improper routing rules between primary and backup mail servers

These loops can fill queues, cause delays, and waste bandwidth. In extreme cases, they can trigger denial-of-service conditions on smaller mail systems.

Loop Detection and DMARCeye

While DMARCeye focuses on email authentication and domain protection, it indirectly benefits from loop detection by analyzing the authentication flow of messages.

When mail servers behave unexpectedly, such as repeatedly forwarding messages, DMARCeye’s reporting and visibility tools help administrators detect anomalies that might indicate looping, forwarding errors, or unauthorized message routing.

By combining authentication insights with infrastructure awareness, DMARCeye helps maintain a clean, efficient mail ecosystem that’s free of routing errors.