When comparing K-9 vs MyMail, 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 MyMail is ranked 14th.
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 Offers 150gb of email storage
My.com account allows storing up to 150gb of emails.
Pro Intuitive swipes for dealing with messages quickly
This app lets you swipe to mark unread, put message in a folder, mark it for later action, or discard it.
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 Caches your passwords even after you uninstall
Their servers may open your mail without you knowing, marking your messages as read without permission.
Con Duplicate notifications
If you still have Gmail installed, you will get a notification from Gmail and MyMail for each message.