E-mail loop

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

E-mail loops are not as common today as in the past, due to changes to e-mail software, both on the client side and the server side, that prevent automatic replies to vacation responses and bounced mail responses.^{[citation needed]}

Costs of an e-mail loop

• Bandwidth: E-mail loops use up limited bandwidth over networks.
• Processing time: E-mail loops will take up processing time, and could slow down other processes.
• Disk space: Automatic e-mails 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 their mailbox.

Causes

• Autoresponders, such as automatic "on vacation" replies
• E-mail bounces due to, for example, exceeding the inbox disk quota
• Replies to indicate that that mail has been delivered
• Replies to e-mail 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.)

Prevention

• The mail system should retain Headers of incoming E-mail while performing any type of auto-forwarding operation.
• Auto Responder: Do not send more than 'x' replies to the same sender.

External links

• 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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