When comparing Nine vs K-9, the Slant community recommends K-9 for most people. In the question“What are the best Gmail clients for Android?” K-9 is ranked 4th while Nine is ranked 10th. The most important reason people chose K-9 is:
K-9 is available for free and licensed under the Apache License version 2.0 with source code available [here](https://code.google.com/p/k9mail/).
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Can adjust color themes for each account.
Pro Supports ActiveSync protocols
Pro Convenient theme options
The black theme is easy on the battery and eyes.
Pro Good customization
But not so many you have to read and dig through documentation to do what you want.
Pro Supports Outlook category colors, Outlook for Android doesn't
Pro Very reliable
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
Pro Lightweight
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.
Pro imap
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.
Cons
Con Not free
Con Not Cross-platform at all
Only has an Android app. No Windows, Mac versions available.
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.