Sisimai: Mail Analyzing Interface

What is a bounced email ?

Bounced Email is a term used to describe an email message that has been rejected by the recipient's email server and returned to the sender due to an error.

It is also commonly referred to as an error email, return email, or undeliverable email. Bounce emails are generated for the reasons listed in the bounce reason list and are returned to the sender with the sender's email address appearing as MAILER-DAEMON or Postmaster.

The first appearance: bouncehammer.jp/en/what-is-bounced-email

Characteristics of Bounced Emails

Bounced emails exhibit distinctive characteristics in their Subject: headers and From: headers, typically following one of a few common formats or variations.

Subject Headers

Bounce emails are automatically generated by the mail server software (Sendmail, Postfix, qmail, etc.) of the sender's or recipient's email server, and the Subject: header of these emails typically follows the following formats:

  • Returned mail: see transcript for details
  • Mail System Error - Returned Mail
  • Non Delivery Notification
  • DELIVERY FAILURE: ...
  • Undelivered Mail Returned to Sender
  • Postmaster notify: see transcript for details
  • Delivery Status Notification (Failure)
  • Warning: could not send message for past 4 hour
  • failure notice

Bounced emails typically have a variety of Subject: lines depending on the mail server software, but they all generally convey that there was a delivery error. Additionally, with the exception of bounce emails generated by some localized mail server software, both the Subject: line and the body of a bounce email are typically written in English.

From Headers

Bounced emails are generated by mail server software (Sendmail, Postfix, qmail, etc.) running on the sender's company, organization, or ISP's mail server, or on the recipient's mail server. However, the From: header is consistent across all bounce emails, and the email address and name fields typically use the following common names:

  • MAILER-DAEMON
  • MAILER-DAEMON@HOSTNAME OF SENDER'S MAIL SERVER
  • MAILER-DAEMON@HOSTNAME OF RECIPIENT'S MAIL SERVER
  • Postmaster
  • Postmaster@HOSTNAME OF SENDER'S MAIL SERVER
  • Postmaster@HOSTNAME OF RECIPIENT'S MAIL SERVER
  • Mail Delivery Subsystem
  • Mail Delivery System
  • Mail Administrator

Message Body

The body of a bounced email varies depending on the mail server software and the specific error. While most bounce emails, particularly those from well-known mail server software like Sendmail, Postfix, and qmail, are written in English, some bounce emails from localized mail server software may provide a clear explanation of the error in Japanese.

Sisimai decode headers and the body of bounce emails to determine if they are bounce emails and the reason for the delivery failure. In the body of a bounce email, the error message itself is important, but the following headers are also particularly important:

  • Status: 5.1.1 <- Various error code
  • Diagnostic-Code: ... <- Error message, response of SMTP server, and so on.

Sisimai is a library which decodes a bounced email, finds a reason of bounce.