K-9 Mail is a free and open source email client for Android.

It is designed as an alternative to the stock email clients included with the platform; it supports both POP3 and IMAP protocols and supports IMAP IDLE for real-time notifications.

[1] The source code was first published to its git repository on October 27, 2008[2] by Jesse Vincent and the first binaries were released to the public on the Google Code site the same month.

[4] On 13 June 2022, it was announced that K-9 Mail had been taken over by MZLA Technologies Corporation, a subsidiary of the Mozilla Foundation with current maintainer Christian Ketterer joining the team, and plans for K-9 Mail to be rebranded as Thunderbird for Android following the completion of a feature roadmap, including sync with Thunderbird on PC, integrating Thunderbird's automated account setup system, message filtering, and improvements to folders.

[5] In the early 2010s it was widely reviewed, and was particularly praised in the media between 2011 and 2013 as a replacement for the default mail application.