Feature Comparison Matrix
Due to the fact most libraries are based on JavaMail / Jakarta Mail, in theory all libraries should be able to do the same things if you start tweaking under hood directly on the related low level components (such as manually setting Session properties). However, since we are comparing Simple Java Mail in the context of its mission, being the easiest to use SMTP library in the world, when a library doesn't support a feature as first class API, it will be marked as not-supported in the matrix below.
Each feature is linked to the appropriate documentation.
All claims below were made objectively with due diligence and in good faith, but if you find any inaccuracies, please raise an issue.
Note 1: In 2023, it is surprising to find only a few major Java libraries for SMTP, next to Simple Java Mail. There seems to be a scarcity of libraries specifically focused on SMTP mailing, with Apache Commons appearing inactive and Spring Mail not receiving enough development as a primary library. Other libraries out there are rarely still active, or are too small and limited compared to Simple Java Mail to be considered a real alternative.
Note 2: As this comparison matrix is taking shape, and the source code of other SMTP libraries is being examined more closely, it is becoming apparent how much Apache Commons Email has been a half-hearted attempt at wrapping JavaMail in a useful API, that lost its momentum completely and ultimately failed as a project and seemingly has been abandoned (last release in 2017 and no meaningful code changes since, last verified Oct 2023). Therefore, remarks on feature differences will be mostly limited simply to "there was an attempt". The fact that they still refer to their Subversion repository on their homepage instead of their GitHub repository is a clear indication of their lack of comittment, let alone some vision for the future. Also, the fact that their home page still refers to JavaMail, which was succeeded by Jakarta Mail in 2019 (and by Angus in 2024), confirms this.