Causes
- 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.)
Read more about this topic: Email Loop