It was a while since the last update to this plugin, and only one outstanding bug was still waiting (issue with the invalid email address using PHPMailer), and with this new release, it is finally fixed.
So, there was a bug in the PHPMailer queue handling where the error catching was done for the sending part, but it was not wrapping the email preparation, where the process can fail if the email address is not valid. This issue has been fixed in both the Pro and Free versions. For the Pro version, other sending engines (REST API) were not having a similar issue as far as I know. Pro version also has an updated SendGrid REST API library.
The next major updates (Pro 5.4 and Free 4.0) will have different requirements and will require WordPress 5.5 (to avoid duplicated PHPMailer handing code – in WP 5.5 new version of PHPMailer is introduced). More about the next update will be available, hopefully, soon.