Email Forwarding Loops

Occasionally, a single user making a mistake can cause a problem. For example, a user whose university account is configured to forward all mail to her Yahoo account. The user goes on holiday. All emails sent to her in her absence are still forwarded to her Yahoo account, which can grow to only 2 MB. When the Yahoo account becomes full, it starts bouncing the emails back to the university account, which immediately forwards it back to the Yahoo account. An email loop is formed that might send hundreds of thousands of emails back and forth, generating massive traffic and crashing mail servers.

There are features of mail server programs that can recognize loops. These should be turned on by default. Administrators must also take care that they do not turn this feature off by mistake, or install an SMTP forwarder that modifies mail headers in such a way that the mail server does not recognize the mail loop.