K-9 Mail

Veteran open-source Android client, now part of Thunderbird.

k9mail.app

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FreeOpen SourceE2E EncryptedAndroidUnited States

Pricing Free and open source, with no ads and no paid tier.

K-9 Mail is a veteran open-source Android client, now maintained by the Thunderbird team and gradually merging into Thunderbird for Android. It handles large mailboxes and multiple accounts smoothly using standard IMAP and POP3. OpenPGP encryption works through the companion OpenKeychain app and it supports the Autocrypt standard for easier key exchange. It is completely free with no ads and no paid tier. If you want a proven, no-nonsense client, it earns its long reputation.

Strengths

  • Free, open source (Apache-2.0), and ad-free
  • OpenPGP via OpenKeychain plus Autocrypt
  • Handles high email volume well
  • Now backed by the Thunderbird project

Considerations

  • Android only
  • Encryption needs a separate OpenKeychain app

Listed in Email Clients

K-9 Mail alternatives

ThunderbirdMozilla's free, open-source desktop mail client.BetterbirdFeature-enhanced Thunderbird fork with no telemetryFairEmailPrivacy-obsessed open-source email for Android.Canary MailPolished paid client with built-in PGP encryption.