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Loop Protection and Znuny

Email systems are essential for seamless communication in today's digital age but are also vulnerable to various issues, including email loops. Read about how Znuny deals with this and how you can configure limits and bypasses for automated messages in Znuny.

An email loop occurs when two or more email servers inadvertently create a cycle of messages, each replying to the other indefinitely. This can happen due to misconfigurations, auto-replies, or other automated processes that respond to each other. Such loops can quickly escalate, leading to server overloads, network congestion, and significant delays in email delivery. To mitigate these risks, implementing robust loop protection mechanisms in email systems is crucial. These mechanisms typically involve monitoring the number of times an email is forwarded, replied to, or bounced back within a specific timeframe and applying thresholds to prevent excessive repetitions.

Loop Protection in Znuny


It's easy to configure loop protection for outgoing emails in Znuny. Default System configurations only allow your system to send 40 automatically generated e-mails daily to any specific address. Sent messages are logged to the database, and the number is validated before sending a message. This limit can be overridden to provide for special use cases where more or less mail is desired.

A search for a loop in the system configuration will lead you to the following options:

  • LoopProtectionLog
    Path to the log file containing the loop protection information. Only written when using the FS module for Loop Protection
  • LoopProtectionModule
    The "Loop Protection" module used by the System (Default: DB)
  • PostmasterMaxEmails
    The maximum number of emails to be sent to any address, with no other explicit limit. (Default: 40)
  • PostmasterMaxEmailsPerAddress

Incoming Loop Protection Mechanisms

One common approach to loop protection is using unique message identifiers and tracking headers, such as the "Received" and "X-Loop" headers, which help identify and terminate potential loops. By examining these headers, email servers can detect repetitive patterns that suggest a loop and take corrective actions, such as discarding duplicate messages or alerting administrators.

Znuny supports parsing these headers and can be configured in a PostMaster filter. You can then add the X-OTRS-Loop = yes to not send any automatic answers to this message. You can also use the setting SendNoAutoResponseRegExp as an additional protection to send no automatic responses to the address.

Other measures

Setting limits on auto-replies and configuring email clients to recognize and halt looping behavior are vital practices. Modern email systems may also incorporate machine learning algorithms to analyze email traffic patterns and predict potential loops before they fully develop. By adopting these strategies, email systems can maintain high reliability and ensure that the detrimental effects of email loops do not disrupt critical communications.