Notes 
K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email. Designed for both novice and power users and offers lots of features for everyone.
Reviewed by Gabriel Bachmann
K-9 Mail
k9mail.app 4.2 (6)
Rate K-9 Mail: No account needed
Threat level
Covered
A covered pick. Anyone can use it as a private drop-in, with no setup or know-how. Enough for most people. Threat levels
Our take
If you have a pile of accounts and you live in your inbox on Android, K-9 Mail handles high volume better than the polished consumer clients, and it is open source with a long track record. It is not itself a private email provider, it is a client, so your privacy still comes from the server you connect it to. The interface looks its age and there is a learning curve; people who want something that just looks nice should consider its successor, Thunderbird for Android.
GitHub at a glance
thunderbird/thunderbird-android Apache-2.0 androidcommunicationemailmozilla
Stars
14,182
+27 this week
Forks
2,881
355 watching
Last commit
today
healthy
Latest release
THUNDERBIRD_24_0
1mo ago, 100+ total
Star history
Issues
83% closed
1,060
open
5,192
closed
+70 / -47
opened / closed, last 4 weeks
Opened vs closed, last 12 weeks
Commits, last 26 weeks
2,466 commits, active 26 of 26 weeks
332 contributors since Feb 2011
Languages
- Kotlin 82.8%
- Java 16.2%
- Shell 0.5%
- Other 0.5%
Updated daily from the GitHub API. Star tracking since Jun 15, 2026.
Website at a glance
k9mail.app
C score 50
Basic website security headers
Graded by Mozilla HTTP Observatory, tested 2d ago
Inspect this site yourself
Measures the security configuration of the tool's own website, not the privacy of the product itself. A strong tool can still score low here.
K-9 Mail alternatives
FairEmail FairEmail is a fully featured, open-source email client for Android that uses standard IMAP/SMTP protocols and supports OpenPGP and S/MIME encryption. It blocks remote image tracking, confirms link opens, and stores no data on third-party servers.
Thunderbird Mozilla Thunderbird is a email client, personal information manager, news client, RSS and chat client.
Canary Mail Canary's security suite is second to none with Seamless End-To-End Encryption, Full PGP Support, Biometric App Lock, On-Device Fetch, No Ads, No Data Mining & Open Source Mail…
Apple Mail Apple Mail is the built-in email client on macOS and iOS, supporting IMAP, SMTP, and S/MIME. Mail Privacy Protection hides the user's IP address and loads remote content via Apple proxy servers to block tracking pixels.
Mailvelope Bring email encryption straight into your browser. We recommend using Firefox for this. It can be used to encrypt and sign electronic messages, including attached files,…
GNOME Evolution GNOME Evolution is a Linux personal information manager combining email, calendar, contacts, and tasks in one application. It uses standard protocols (IMAP, SMTP, CalDAV, CardDAV) and supports native OpenPGP and S/MIME encryption without plugins.
K-9 Mail license, in plain English
Apache-2.0 Permissive
Permissive like MIT, with an explicit patent grant and a requirement to flag any changes you make.
Permits
- Commercial use
- Modification
- Distribution
- Patent use
- Private use
Requires
- License and copyright notice
- State changes
Does not provide
- Trademark use
- Liability cover
- Warranty
Why it matters: Permissive licensing lets anyone reuse this, including inside closed products. That is freedom to build on, but no guarantee that downstream copies stay open.
Plain-language summary of the project's license, not legal advice. Read the full text for the exact terms.