Mozilla today announced a new email client for Android devices called K-9 Mail. The mobile app is being launched as part of Mozilla's Thunderbird family of open-source desktop email and chat applications.

K-9 Mail is not new. The source code for it was first published 2008, and MZLA Technologies acquired it and its trademarks, and it hired the open source project maintainer of K-9 Mail as a full-time developer to improve the app and to help gradually transition it to Thunderbird mobile.

MZLA Technologies, a subsidiary of the Mozilla Foundation, said it is also currently evaluating the development of a K-9 email client for iOS devices.

The development team for MZLA Technologies plans to improve the email feature set to more closely resemble that of its Thunderbird desktop email client.

Those planned improvements include account setup using Thunderbird account auto-configuration, as well as folder management, support for message filters, and synchronization between desktop and mobile versions of Thunderbird, Mozilla said. (K-9 Mail is available for download through these various sites.)

"Ultimately, K-9 Mail will transform into Thunderbird on Android, once certain development milestones have been achieved which align it closely with Thunderbird's feature set and visual appearance," K-9 Mail Project Maintainer Christian Ketterer said in a blog post. (Ketterer is better known in the Mozilla open-source developer community as cketti.)

Thunderbird is a free and open-source email, calendaring, newsfeed, and chat client with more than 20 million active monthly users across Windows, macOS, and Linux.

Currently, Android mobile users have a number of email and chat options, including Google's Gmail, Microsoft Outlook, and ProtonMail, an end-to-end encrypted email service.

Ryan Lee Sipes, Thunderbird Product Manager, said working with the Thunderbird email team to develop the new mobile client made more sense than developing a mobile client from scratch. "Joining the Thunderbird family allows K-9 to become more sustainable and gives us the resources to implement long-requested features and fixes that our users want," Sipes said. "In other words, K-9 Mail will soar to greater heights with the help of Thunderbird."

Mozilla plans to offer Firefox Sync as one option to allow users to securely sync accounts between Thunderbird and K-9. The feature is expected to be implemented in the summer of 2023.

Andrew Cornwall, a senior analyst with research firm Forrester, called the K-9 Mail annoucement "significant."

"K-9 was one of the first third-party email clients on Android, and it developed a large user base among users who had to keep track of multiple inboxes. About ten years ago it was among the most recommended third-party email clients for Android, and a lot of long-time users have stuck with it. As of today, it's listed as having 5M+ downloads on Google Play," Cornwall said.

Development of K-9, however, stalled and the code was basically in "maintenance mode," he noted. In January, a new 6.000 release went out to the Play Store, a move that surpised many users. The release included an updated user interface that focused on a single inbox rather than multiple inboxes. Instead of being welcomed, the in focus upset many existing users, Cornwall said.

Another concern for existing users of K-9 Mail was its support for OAuth2, an authorization protocol that allows the client to access email resources of the user without sharing passwords.

"This has been a perennial complaint (since at least 2015) and the issue has now been forced: Google has disabled the traditional way that most K-9 Mail users used to access their Gmail," Cornwall said. "Users are left with workarounds involving two-factor authentication for Gmail, and no real workaround for Outlook when organizations require an OAuth2 login."

Overall, Mozilla's adoption of K-9 Mail will be welcome to its existing users, many of whom use Thunderbird on their desktops for a similar multiple-inbox workflow, Cornwall said.

"Today, some K-9 Mail users have switched to using K-9 Mail for their non-Gmail mail and the Google Gmail app to check Gmail. The thought of a new Google Play release from Mozilla with Oauth2 fixes may be the encouragement that existing users need to stick around," he said.

K-9 will have a similar look and feel to Thunderbird's current desktop app, with the exception of certain functions.

Thunderbird for Android initially will not support calendars, tasks, feeds, or chat like the desktop app, cketti wrote in a Q&A included as part of his blog post. Ketterer said the developer team is first working on "an amazing email experience," and is considering the best way to provide Thunderbird's other functionality on Android. "But currently [we] are still debating how best to achieve that."

For instance, he wrote, one method to be able to use Thunderbird's desktop email feature set is to simply sync calendars, and then users can use their preferred calendar application on their device.

"But we have to discuss this within the team, and the Thunderbird and K-9 communities, then decide what the best approach is," Ketterer wrote.