Hi,
I think I just had the same issue.
It doesn’t show an error, but it neither sends the mail.
I’ve solved it twice this week doing the following steps:
1. Revoke access of the application
2. Create a new OAuth Client ID on Google Cloud Platform Console
3. Delete all the info on the pluggin settings and saving
4. Re-write all the information with the new OAuth Client ID and save settings
5. Grant access again
Once this is done, I’m able again to send mails.
What do you think about it? Could it be a minor problem, a compatibility problem with another plugin or a configuration problem maybe?
Thank you.
Best regards.