Simple Java Mail is a robust library, but there's always room for improvement: suggest anything you can think of!
The Simple Java Mail GitHub repository and the issue tracker are used to discuss feature requests and issues.
You can also refer to the API documentation.
If you need more general help, you'll usually get an answer by posting a question on StackOverflow. And you can always post an issue in the issue tracker, I get an email as soon as something gets posted.
You can find more about me on bennybottema.com. At the bottom of the page you can shoot me an email if you want a quick answer.