I'd like to know the relation between K-9 and Thunderbird. I couldn't register to https://forum.k9mail.app to chat as no registration email was sent to me during couple of days since I tried to sign up. I don't want another chat app, like Matrix, I prefer forums. Why thunderbird-android on Github is signed as "fka K-9"? Is K-9 becoming Thunderbird? So why there is Thunderbird beta? Both apps are said to be developed by the same developer on Google Store. I need some additional info before I switch to another all-except-gmail-bloatware email client on a new device.