It could be related to some dmarc, SFP config, conflict with other plugins
These issues are specific to your environment and not a general problem.
The plugin is active in more than 300,000 sites.
Even if not updated for eight months, there isn’t a specific update interval that determines if the plugin can have issues or not.
The plugin can stay like this for years and work the same way he worked on his latest release (except for fixes or compatibility issues).
I would appreciate it if you could convert this thread to a support thread as it is more suitable for this case and not leave it as a low-star review.
If not, it’s still ok, and you have the right to express your feeling as you see fit.