The branch 3.X of GNU mailutils fixed the following deficiences found in versions 2.X:
- Provide a consistent and effective interface for stream I/O.
- Reorganize the codebase to use streams API, thereby improving performance.
- Rewrite the filter API to improve its performance and to allow for easy creation and chaining of filters.
- Improve logging and debugging support.
- Provide API for word splitting and variable substitution instead of the obsolete argcv module.
- Rewrite the client support for IMAP4.
- Provide an extensive testsuite for Mailutils API as well as for the particular applications (in 2.X, the testcases were based on DejaGNU and were mostly non-portable).