The latest interface update has seen mixed reviews. K-9 Mail remains available, but users who have installed it will experience transformations in the coming months and years to align the email application closely with Thunderbird's brand and feature set. The client will undergo visual changes to align it closely with Thunderbird.

K-9 Mail will turn into Thunderbird for Android eventually, but this won't happen overnight. The team expects to introduce synchronization support in the Summer of 2023. Thunderbird will 'devote financial and development resources to improving K-9 Mail', including an improved account setup, something which Thunderbird received already in a recent update, improved folder management, support for message filters, and the option to sync data between desktop and mobile clients. Ketterer has joined the Thunderbird team already, 'bringing along his expertise and experience with mobile platforms', and the K-9 Mail application. K-9 Mail 'aligns perfectly with Thunderbird's values of using open standards, respecting the user, and enabling power users with unmatched customization' according to Sipes. The idea to collaborate with an existing open source email project for Android was born in 2018, when Sipes started talking to Christian Ketterer, the K-9 Mail project maintainer, about potential collaborations. The development of an Android email client from the ground up was one of the options that the team discussed, but it would have been a large project that would have required considerable effort and resources.

The Android version enables Thunderbird users to sync data between desktop versions and their mobile Android devices. The release would fill a gap, as Thunderbird has been available for desktop operating systems only up until now. Thunderbird product and business development manager Ryan Lee Sipes confirmed in May 2022 that Thunderbird would become available for Android soon.

K9 mail android