Sisimai | Library to decode bounce mails | Sisimai: Mail Analyzing Interface

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 versions of Sisimai 5.2.1 and 4.25.17, released on Wednesday, 12th March 2025. The Go version of Sisimai is available since Sisimai 5.2.0. The changes in this release are minimal, updates for that NTT DOCOMO (Major Japanese mobile carrier) no longer returns a bounce mail due to domain rejection or similar email settings, but instead were being delivered to the spam folder after March 13th. This change is considered to have a significant impact in Japan, hence the simultaneous release of the 4 series. For details on other changes, please refer to the respective release notes (v5.2.1 or v4.25.17)

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.

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Detect Bounce Reasons

Sisimai can decode 36 bounce reasons, including common errors such as "userunknown", "mailboxfull", and "rejected".

Details

Data Structure

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

Details

59 Decoding Engines

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

Details

Sisimai decodes over 110 million bounce emails every month worldwide.

About the Logo only showcase

vs. bounceHammer

In the summer of 2014, Perl version Sisimai was developed to solve numerous critical issues related to the parsing accuracy of bounceHammer. Two years later, on February 3, 2016, the Ruby version of Sisimai was also released.

Both the Perl and Ruby versions of Sisimai continue to parse 110 million bounce emails worldwide every month (according to our research). On February 2, 2024, Sisimai 5.0.0 was released, capable of detecting modern bounces, including domain authentication (SPF, DKIM, DMARC, ARC) errors and reputation-related errors. On August 16 of the same year, it celebrated its 10th anniversary since its release.

And on February 25, 2025, Go version Sisimai, an implementation in a non-scripting language, was released as the third Sisimai. Go version Sisimai, like the Perl and Ruby versions, minimizes dependencies and is characterized by the ease of deployment and execution speed ( 12.2 times faster than bounceHammer, 4.4 times faster than Perl version Sisimai v5 ), which are advantages of compiled binaries.

Analytical Precision Ratio

100%

59%

The Speed Of Decoding 1000 Emails (Go version)

0.3s

4.24s

Detectable Bounce Reasons

The Number Of MTA/ESP Modules

Dependent External Modules (Go version)

Celebrating

10

Years of Release

Can Detect

36

Bounce Reasons

Have

59

Engine Modules

Only

0

External Modules Required

Download

The latest version is 5.2.1, released on Wednesday, 12th March 2025. As of February 2025, the Go version of Sisimai is available from v5.2.0.

Sisimai can run on any OS where Perl or Ruby works, such as *BSD, macOS, and Linux. 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 - 5.40.0
CPAN

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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

Download

Go Version

Release version
Sisimai 5 Only
Go 1.17 -

Download

Fork on GitHub

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

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Support

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