When comparing K-9 vs Nine - Exchange ActiveSync, the Slant community recommends K-9 for most people. In the question“What are the best e-mail clients for Android? ” K-9 is ranked 2nd while Nine - Exchange ActiveSync is ranked 3rd.
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 Great UI
The stack approach is one of this apps strongest points, as it provides the fastest access to mail and is the cleanest to sort through.
Pro Really fast
Pro Many widgets
Pro Incredible feature set
Calendar, contacts, notes, tasks.
Pro Very reliable
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).
Con Dead app
Con Doesn't support Gmail or Yahoo
Supported servers include
Exchange Server 2003 SP2/SP3 and above
Google Apps account
Although many basic servers are supported, it makes no sense that some of the most popular ones would be left out.
At $9.99 (down from $20), they are priced much higher than their competition.