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.
Costs of an email loop 
- Bandwidth: Email loops use up limited bandwidth over networks.
- Processing time: Email loops will take up processing time, and could slow down other processes.
- Disk space: Automatic emails are usually stored in the mailboxes of participants.
- Human Time: Network administrators may have to intervene to fix the problem, or clean up mailboxes. Also the mailbox user/owner will have to delete the numerous responses in order to clean up the mailbox.
- Autoresponders, such as automatic "on vacation" replies
- Email bounces due to, for example, exceeding the inbox disk quota
- Replies to indicate that that mail has been delivered
- Replies to email read-receipts
- Misconfigured email servers that try to deliver messages to systems that pass the message along to another host, with a loop leading in a circle. (Modern mail systems will detect mail forwarded back and forth between two hosts, but a routing loop involving three hosts is much harder to detect.)
- The mail system should retain headers of incoming email while performing any type of auto-forwarding operation.
- Auto Responder: Do not send more than 'x' replies to the same sender.
- Vacation E-Mail Loop
- The Free Dictionary
- RFC 2919
- What does the "too many hops" error mean when an email message I send fails?
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