Upgrading to 5.0.0? Read the migration notes.
Simple Java Mail is open source, and you can use it for free in any personal or commercial product under the Apache v2 license.
Simple Java Mail is available in Maven central as dependency and you can also download the jar distribution directly from there.
Simple Java Mail is released under Apache v2 license
In short this means you can do whatever you want as long as you include the same license, the original (possibly ammended) notice, copyright and that you state your changes.
You can visit the repository Simple Java Mail on gitHub.