Thunderbird's Android app, which is actually the K-9 Mail project reborn, is almost out. You can check it out a bit early in a beta that will feel pretty robust to most users.
Thunderbird, maintained by the Mozilla Foundation subsidiary MZLA, acquired the source code and naming rights to K-9 Mail, as announced in June 2022. The group also brought K-9 maintainer Christian Ketterer (or "cketti") onto the project. Their initial goals, before a full rebrand into Thunderbird, involved importing Thunderbird's automatic account setup, message filters, and mobile/desktop Thunderbird syncing.
At the tail end of 2023, however, Ketterer wrote on K-9's blog that the punchlist of items before official Thunderbird-dom was taking longer than expected. But when it's fully released, Thunderbird for Android will have those features. As such, beta testers are asked to check out a specific list of things to see if they work, including automatic setup, folder management, and K-9-to-Thunderbird transfer. The beta will not be "addressing longstanding issues," Thunderbird's blog post notes.
Launching Thunderbird for Android from K-9 Mail's base makes a good deal of sense. Thunderbird's desktop client has had a strange, disjointed life so far and is only just starting to regain a cohesive vision for what it wants to provide. For a long time now, K-9 Mail has been the Android email of choice for people who don't want Gmail or Outlook, will not tolerate the default "Email" app on non-Google-blessed Android systems, and just want to see their messages.