When comparing K-9 vs Gmail, 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 Gmail is ranked 10th.
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 Best support for Gmail features
Since both the Gmail Android app and Gmail itself is made by Google, the app supports all features Gmail offers (like labeling/starring) and gets new features implemented early - usually as soon as they are announced.
Pro Incredibly fast, advanced & predictive search
Gmail brings much of the functionality and power of Google's search engine to the app's search. You can use operators such as label:, is:, OR, has: among others, define date ranges, limit search to specific groups and so on. And, you can do it all amazingly fast. The search will also try to predict what you're trying to search for before you've even finished your query and offer up those results.
Pro Google Drive integration
The app allows inserting files from Drive straight to email or uploading attachments to Drive before sending them out. It will also notify if the person you're sending the file can open the file or if you should change the sharing settings for that file beforehand. Finally, you can choose which one of your Google accounts you want visible in the app.
Pro Google Calendar integration
Pro Comes bundled with Android for free
Not only is the application free, it comes bundled with the Android OS by default so you don't even have to bother downloading it.
Pro Threaded conversations
Pro Safely automatically displays images
The app does not require a confirmation to show images as Google now hosts the images on their own servers making it safe. You can still disable the automatic displaying of images in the settings if you wish.
Pro Tabs to automatically organize your e-mail
Gmail offers the ability to automatically organize your mail into specific categories:
Each of the categories can be disabled and you can move your mail around from category to category or set your own rules which sender should be put in which category.
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 Sorting mail by Unread is unintuitive
Nearly every other mail app has a clear way to sort mail by Unread. In Gmail app you can do that only by typing “label:unread” in the search field.
Con Very limited HTML composing
Composing of HTML formatted mails is very limited. No inline images and only some basic text formatting options
Con Too brights, no dark theme
Con Advertising has been added to the client
Con Too heavy
It should be lightweight.
Con Limited notification buttons
Con Changed theme and can't change it back!
Did it without permission and it looks terrible! Washed out look, solid white background with black lame fonts.