Mozilla Thunderbird has announced that it is collaborating with K-9 Mail to bring its email client to Android. The project manager for Thunderbird, Ryan Lee Sipes, had recently publicized that this project was on its way. Now, K-9 Mail's maintainer Christian Ketterer, known as "cketti", has also announced that K-9 Mail will soon become Thunderbird.
While only for Android users right now, Mozilla has stated that support for iOS is being considered too. Furthermore, both cketti and Sipes have decided to keep the project open-source; therefore, due to shared funding, any donations sent to K-9 Mail will also be used by Thunderbird.
Thunderbird and K-9 Mail users will be allowed to sync email data from their existing accounts through Firefox Sync from summer 2023. Other prospective features include better message organization through message filters and folders, as well as account setups through auto-configuration by Thunderbird. The app interface for K-9 Mail will match Thunderbird's desktop application to some extent, however, it will not have features like calendars and tasks for now.
All file archives and source code repositories will shift to Thunderbird on GitHub. Moving forward, cketti stated that the K-9 mail app might move to some other publisher account from Google Play in order to unify the project under a clearer account name too. If you want to actively participate in the future of K-9 Mail's revamp to Thunderbird, you can install the app from Google Play. There isn't a concrete end date for this merger yet but it will likely happen in staggered manner over the course of the next few months.