The 0.4 release brought a number of significant improvements in terms of supported capabilities, flexibility and performance.
The 0.5 release addressed a number of important issues discovered since the 0.4 release. In particular it improved Mime4j ability to deal with malformed data streams including those intentionally crafted to cause excessive CPU and memory utilization that can lead to DoS conditions.
The 0.6 release brought another round of API enhancements and performance optimizations. As of this release Mime4j requires a Java 1.5 compatible runtime.
The next release will be version 0.7. The main focus of the development efforts will be on optimization of MIME header parsing and further enhancement of DOM capabilities.