When comparing K-9 vs FairEmail, the Slant community recommends FairEmail for most people. In the question“What are the best e-mail clients for Android? ” FairEmail is ranked 1st while K-9 is ranked 2nd.
Ranked in these QuestionsQuestion Ranking
Pro No privacy concerns
Pro Free an open source
K-9 is available for free and licensed under the Apache License version 2.0 with source code available here.
Pro Complete PGP encryption
It's possible to complete encrypt email communications via PGP using K-9.
K-9 mail does not use too many system resources when using the app, which allow it to be fast and lightweight.
Works well with locally maintained IMAP server.
Pro Material UI version
There is a fork of K-9 available on Google Play that uses Material design, improves the interface a lot.
Pro Excellent email threading
Threaded email support is better than most.
Pro Optimized for phone and tablet
On a tablet the UI looks a little different but given the extra real estate is logical.
Options to enable UI widgets a plus, can be enabled on a tablet and disabled on a phone.
Pro Open source
Pro Multiple account support
Multiple identities per account, notifications per account or contact.
Pro Fast and customizable
Privacy features are excellent.
Pro Identities support
Pro Two way synchronization
Pro Flat conversation threading
Pro Encryption support
Supports standard OpenPGP encryption for total privacy.
Pro Can show plain text of html mails
Pro Support for safety and low-bandwidth
Allows to configure maximum size of email downloaded immediately.
Con Noisy UI
The user interface is bland and tries to cram a lot of information on screen, making it unpleasant to use.
Con Emails received as forwarded EML attachments are not natively supported
When you receive an email which was forwarded as an EML attachment, K-9 Mail is not able to open it natively. It opens it using a third-party application registered for this mime-type on the device, if any. The Gmail email client app is able to read the EML attachments, thus you need to have it installed as well (or any other EML capable app).