I tried logging in using my Google account, but I got this:
"Access blocked: tubemail.io has not completed the Google verification process
p.c.frankplads@gmail.com
tubemail.io has not completed the Google verification process. The app is currently being tested, and can only be accessed by developer-approved testers. If you think you should have access, contact the developer.
If you are a developer of tubemail.io, see error details.
It's not open source, but free to use (with no ads or catches)