What is a Mail Loop (Email Loop)?

What Causes an Email Loop?

Mail loop, also referred to as an email loop, is a simple infinite loop phenomenon. These loops occur when mail servers, scripts or email clients generate automatic email replies or responses. When one email automatic response is initiated– possibly an event triggered email linked to a user’s birthday it triggers another automatic response, such as an automatic “on vacation” autoresponder, this is an example of a mail loop.  This process of continued automated communication will go on until one mailbox is full or reaches its mail sending limit — or one user stops their automatic communication.  Email loops are often caused accidentally, but these loops can also be malicious denial of service attacks. Luckily, these loops are rare today as email software can prevent responses to autoresponders or bounced mail.

How to Prevent an Email Loop?

While many email loops are caused accidentally, they can still be costly to users. These loops affect network bandwidth, processing time, email program’s disk space, and sometimes the schedule of network administrators as they have to clean up the mailboxes.  These loops are also easily prevented, however. Often, the system can limit the number of auto responses to the same sender — effectively stopping the loop before it gets out of control.