What is Sisimai ?

Sisimai is a library that decodes complex and diverse bounce emails and outputs the results of the delivery failure, such as the reason for the bounce and the recipient email address, in structured data. It is also possible to output in JSON format.

The latest stable version is Sisimai 5.1.0 , released on Monday, 1st July 2024. The main changes in this version are the complete removal of v4 compatibility features, DMA: DragonFly Mail Agent bounce email support, and the update of error codes for Yahoo Inc. and others.
For details on the all changes in Sisimai 5.1.0, please see the release notes. Please check Differences with Sisimai 4 page for the breaking changes in Sisimai 5.
Sisimai versions 4.25.14p11 and earlier contain a regular expression vulnerability (ReDoS: CVE-2022-4891). If you are using one of these versions, please upgrade to v4.25.14p12 or later.

Get Started !

Sisimai is a library to decode bounce mails for UNIX mbox format files and those in Maildir/ directories.

Get Started

Detect Bounce Reasons

Sisimai can decode 34 bounce reasons, including common errors such as "userunknown", "mailboxfull", and "rejected".


Data Structure

The data of bounce emails decoded by Sisimai is structured and can also be output in JSON format.


73 Decoding Engines

Sisimai can decode bounce emails from open-source MTAs, commercial MTAs, email appliances, and well-known email services.


Sisimai decodes over 110 million bounce emails every month worldwide.

vs. bounceHammer

In the summer of 2014, the Perl version of Sisimai(pronounced as 'shi-shi-mai') was developed to solve several critical problems with the decoding accuracy of bounceHammer, including: Inability to decode multiple bounces in a single email, Large number of dependent modules, some of which are no longer maintained, Inability to work with Perl 5.16 or later.

Sisimai is now about 2.2 times faster than bounceHammer and has twice the decoding accuracy. The Ruby version of Sisimai was released on February 3, 2016.

And on February 2, 2024, the 10th anniversary of the release of Sisimai, we released Sisimai 5.0.0, which can detect modern bounces such as those related to domain authentication (SPF, DKIM, DMARC, ARC) and reputation-based errors.

Analytical Precision Ratio



The Speed Of Decoding 1000 Emails



Detectable Bounce Reasons



The Number Of MTA/ESP Modules



Dependent External Modules





Years of Release

Can Detect


Bounce Reasons



Engine Modules



External Modules Required


The latest version is 5.1.0, released on Monday, 1st July 2024.

Sisimai can run on any OS where Perl or Ruby works, such as *BSD, macOS, and Linux. Sisimai 4.22.5 and later may run with ActivePerl on Windows. .

It is important to review the release notes and the differences with Sisimai 4 page before installing or upgrading, as there are several breaking changes in Sisimai 5.0.0 that may affect your use of the software.

Perl Version

  • Release version
  • v4: Perl 5.10.1 - 5.24.4
  • v5: Perl 5.26.0 - 5.38.2
  • CPAN

Ruby Version

  • Release version
  • v4: Ruby 2.1.0 - 3.3.0
  • v5: Ruby 2.4.0 - 3.3.0
  • JRuby 9.2 - 9.4 #267

Fork on GitHub

  • Development Version
  • p5-sisimai (Perl)
  • rb-sisimai (Ruby)
  • 4-stable, 5-stable


Sisimai is an open-source library licensed under the two-clause BSD license, so anyone can use it freely. However, Cubicroot Corporation Limited, the developer of Sisimai, offers products and services to support the following:

  • Building an environment for bounce mail decoding
  • Developing scripts around Sisimai
  • Designing and managing a database for storing decoded data
  • Continuous bounce management
  • Migrating from the bounce mail decoding system bounceHammer
Read More About Support