A Mail Transfer System Selectively Restricting a Huge Amount of E-Mails

With the wide use of e-mail, it has become commonly expected that e-mails sent are delivered to recipients immediately. It is not rare that a huge amount of emails are posted to a mail transfer system in very short time unintentionally, due to misconfiguration such as an e-mail loop. Such e-mail flood increases the load of the mail transfer system, and causes serious delays in e-mail delivery of the system. We propose a mail transfer system with resiliency against e-mail flood. In the proposed system, the number of each pair of envelope-from and envelope-to is counted. If the number of sessions including a specific pair (envelope-from, envelope-to) during a short period exceeds a pre-specified threshold, we regard this pair as a cause of a huge amount of e-mails, and reject e-mails including this pair after that. By this way, a huge amount of e-mails are selectively restricted without affecting transfer of ordinary e-mails.