According to the Thunderbird team, talks between Thunderbird product manager Ryan Lee Sipes and K-9's lead maintainer Christian Ketterer over a possible collaboration for a Thunderbird email app started in 2018.
Fast forward to 2022, the two have decided the best path forward is for K-9 to simply join Thunderbird rather than Thunderbird building its own app from scratch.
"Many Thunderbird users have asked for a Thunderbird experience on mobile, which we intend to provide by helping make K-9 amazing (and turning it into Thunderbird on Android). K-9 will supplement the Thunderbird experience and enhance where and how users are able to have a great email experience. Our commitment to desktop Thunderbird is unchanged, most of our team is committed to making that a best-in-class email client and it will remain that way," Thunderbird said.
This means K-9 will take on the Thunderbird name and branding. But first, K-9 Mail will need to be aligned with Thunderbird's feature set and visual appearance.
"To accomplish that, we'll devote finances and development time to continually improving K-9 Mail. We'll be adding brand new features and introducing quality-of-life enhancements," the Thunderbird team said in a blogpost.
K-9 isn't a hugely popular email app on Google Play but has had five million downloads from the Google Play Store.
The roadmap for K-9 Mail currently includes: creating account setup using Thunderbird account auto-configuration; improved folder management; support for message filters; and syncing between desktop and mobile Thunderbird.
While Thunderbird acknowledges there's been demand for a Thunderbird app on iOS, the project only says in an FAQ that it is currently "evaluating" that option.
The team says it remains committed to desktop Thunderbird and that K-9 Mail will be "brought in-line with Thunderbird from a feature perspective, and we will ensure that syncing between Thunderbird and K-9/Thunderbird on Android is seamless."
The Thunderbird team also plans to offer Firefox Sync as as an option to sync accounts between Thunderbird and K-9 Mail. This should be implemented in summer 2023.
The project is also exploring what Thunderbird features to bring to the Android app, such as support for calendars, tasks, feeds and chat.