Identifying and Fixing Email Loops

Email loops are a disruptive and potentially dangerous condition in which messages are continuously forwarded or bounced between two or more servers, often consuming system resources, generating large volumes of unnecessary email traffic, and in severe cases, overwhelming servers or triggering spam detection systems. These loops usually arise due to misconfigurations in mail routing, automatic forwarding, incorrect MX records, or logic flaws in email filters and aliases. Because they typically involve multiple systems and recursive behavior, diagnosing and resolving email loops requires a deep understanding of mail flow, routing policies, and DNS configurations—especially those involving MX records.

A classic email loop scenario occurs when two mail servers are configured to route undeliverable mail to each other as fallback destinations. For instance, if server A cannot deliver a message and is set to relay to server B, and server B in turn is configured to relay similar undeliverable messages back to server A, a loop can occur. Each server believes it is forwarding mail to a valid endpoint, but instead the message continues bouncing back and forth indefinitely or until a hop limit is reached. Most SMTP servers have a built-in maximum hop count—often 30—to prevent infinite loops, at which point the message is bounced with a “too many hops” or “mail forwarding loop detected” error. Despite this safeguard, the loop may have already generated dozens of redundant messages, consuming bandwidth and processing power.

MX record misconfigurations often play a central role in initiating these loops. For example, if multiple domains point to the same MX server without proper local delivery routing rules, the server may not know how to handle email for one or more of those domains. When it receives such a message, it might forward it based on a catch-all rule, forwarding configuration, or fallback relay policy. If the recipient server then performs a similar check and sends the message back, a loop begins. This can also occur in hybrid environments where cloud services and on-premises systems coexist, and MX records route messages through both. Improper failover settings, circular smart host definitions, or inconsistent user routing between systems can easily create a situation where mail is endlessly bounced.

Email forwarding rules are another frequent cause of loops. A user may set up a forward from one account to another, not realizing that the destination account has an auto-reply or a return-forward in place. This can be particularly problematic with shared mailboxes, help desk systems, or distribution groups, where incoming messages trigger auto-responses that are sent back to the original sender—who is also configured to forward messages again. In environments with catch-all email configurations or complex aliasing, such recursive behavior is difficult to track and often overlooked until performance issues or user complaints begin to surface.

To identify an email loop, administrators should start by examining the full headers of affected messages. The “Received” headers will usually repeat the same set of hostnames or IP addresses multiple times, indicating that the message is cycling through the same path repeatedly. The number of “Received” headers may exceed normal expectations, or the headers may contain identical timestamps, suggesting automated bouncing. In some cases, the message may include diagnostic information in the body or subject line, especially if a mail gateway or spam filter has detected the loop and inserted a warning such as “possible mail loop detected” or “message bounced due to excessive hop count.”

SMTP logs on affected servers provide another critical source of evidence. Repeated delivery attempts for the same message ID or identical sender-recipient combinations that occur in rapid succession point to looping behavior. Queue inspection may also reveal a backlog of messages stuck in retry cycles with similar error codes, such as 550 5.4.6 or 554 too many hops. In environments where mail passes through multiple security gateways or relay systems, logs from each system must be correlated to identify the full scope of the loop and understand where it originated.

Once a loop is confirmed, resolution involves correcting the underlying configuration that enabled the recursion. In cases involving MX record misrouting, administrators should ensure that each domain’s MX records point only to systems explicitly configured to handle mail for that domain. Additionally, those mail servers must be aware of all valid recipient domains and local delivery rules. It is critical to avoid configurations where a server is asked to accept mail for a domain it does not serve and then attempts to relay the message elsewhere. SPF, DKIM, and DMARC records should be reviewed as well, as authentication failures combined with forwarding or bouncing can exacerbate loop conditions and cause messages to be blocked by receiving systems.

If the loop involves forwarding rules at the user or mailbox level, administrators must review mail client settings, server-side mail rules, and auto-reply configurations. Forwarding should always include safeguards to prevent recursion, such as only forwarding once per message or suppressing auto-replies to messages marked as automated. In enterprise environments using Exchange, Microsoft 365, or Google Workspace, administrators can configure mail flow rules to detect and prevent auto-forwarding to known risky destinations or to break loops involving auto-replies. Implementing loop detection mechanisms at the gateway level, which inspect header patterns and reject messages with repeated paths, adds another layer of protection.

After implementing fixes, continued monitoring is essential to ensure the loop has been broken. Queue lengths should return to normal, message throughput should stabilize, and no new recursive headers should appear in email traces. If possible, test scenarios should be recreated in a controlled environment to confirm that the changes are effective. Documentation of the issue and the corrective actions taken should be maintained for future reference, especially in complex or multi-tenant infrastructures where configuration changes can have far-reaching effects.

Email loops are a classic example of how small misconfigurations in routing or DNS can lead to outsized disruptions in system performance and reliability. They can consume network resources, damage domain reputation, and delay or destroy legitimate communications. Through careful analysis of mail flow, diligent configuration of MX records and routing policies, and proactive implementation of loop detection controls, administrators can prevent these problems and ensure that email systems operate efficiently and predictably, without falling into recursive traps.

Email loops are a disruptive and potentially dangerous condition in which messages are continuously forwarded or bounced between two or more servers, often consuming system resources, generating large volumes of unnecessary email traffic, and in severe cases, overwhelming servers or triggering spam detection systems. These loops usually arise due to misconfigurations in mail routing, automatic…