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 3rd.
Ranked in these QuestionsQuestion Ranking
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 No privacy concerns
K-9 mail does not use too many system resources when using the app, which allow it to be fast and lightweight.
Pro Complete PGP encryption
It's possible to complete encrypt email communications via PGP using K-9.
Works well with locally maintained IMAP server.
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 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 Open source
Pro Multiple account support
Multiple identities per account, notifications per account or contact.
Pro Actively maintained
It is actively maintained. The used libraries are up to date and bugs are fixed quickly.
There are no trackers within the app and trackers sent with mails are disabled by default. This app lets you read and compose emails with your privacy in your very own hands.
Pro Fast and customizable
Privacy features are excellent.
~ 20 MB.
Pro Identities support
Pro Two way synchronization
Pro Flat conversation threading
Pro Encryption support
Supports both OpenPGP and S/MIME encryption for total privacy.
Pro Support for safety and low-bandwidth
Allows to configure maximum size of email downloaded immediately.
Pro Can show plain text of html mails
Pro Show links before opening
Some HTML mails may show different links and link texts, this app however will show you the real link before opening, protecting against phishing and scams
Pro Recognize spoofing internationalized domain names
Some domain names may use non-ASCII characters looking similar to the latin alphabet (e.g. some cyrillic characters). This app will show you the punycode representation of the domain, if that is the case and thus protect you against phishing and scams.
Pro Reformat the HTML mail for increased privacy
Some HTML mails may include ressources that can track you. This app doesn't load them by default, but reformats the text, increasing both privacy and readability.
Pro Autoconfiguration with open standards
There are open standards for any email provider to show how an email client should connect to the server. This app supports both RFC 6186 as well as Thunderbird Autoconfiguration standards.
Pro OAuth support for most common mail providers
Supports authenticating with the mail server using OAuth, thus increasing security.
Pro Low RAM usage at the right setings
Con Dead app
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).
Con Noisy UI
The user interface is bland and tries to cram a lot of information on screen, making it unpleasant to use.