An email loop is an infinite loop phenomenon, resulting from mail servers, scripts, or email clients that generate automatic replies or responses. If one such automatic response triggers another automatic response on the other side, an email loop is created. The process can continue until one mailbox is full or reaches its mail sending limit. In theory, the email loop could last indefinitely. Email loops may be caused accidentally or maliciously, causing denial of service. Although rare, email loops involving more than two participants can also occur.

Email loops are not as common today as in the past, due to changes to email software, both on the client side and the server side, that prevent automatic replies to vacation responses and bounced mail responses.