MailNews stands for "Mail and Newsgroups". MailNews is the back end of the messaging parts of SeaMonkey and Thunderbird. It provides a number of functions and capabilities, including:

The MailNews code lives in the mailnews/ directory of comm-central.

AsyncTestUtils extended framework
The AsyncTestUtils Extended Framework is one mechanism for testing the MailNews component of Thunderbird. See MailNews automated testing for a description of the other testing mechanisms.
Leak And Bloat Tests
This page describes how to perform tests that measure memory leaks and bloat for MailNews and its sub-components.
MailNews automated testing
Automated testing is an increasingly significant and important part of MailNews development. It allows reviewers to quickly pick up regressions in patches and helps developers to think about different cases.

See pages tagged MailNews for additional MailNews-specific information.