When comparing Google Inbox vs K-9, 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 Google Inbox is ranked 8th.
Ranked in these QuestionsQuestion Ranking
Pro Good-looking interface
Inbox follows Google's Material Design guidelines, making for an intuitive and smart looking interface. Bold colors with clear text and intuitive actions allow for one of the better looking in house Google apps on Android.
Pro Intelligent categorization
Inbox tries automatically bundling similar messages together. For example, Inbox will create bundles for travel information, receipts, promotions as well as group messages temporally. It enables also managing these bundles of messages together - they can all be swiped away together.
Pro Can snooze emails
It's possible to "hide" a message until a specific date and time or location, which gets the message out of your way until you're ready to deal with it. Swipe left and you can have the email disappear for a day, a week, a month, etc.
Pro Helps deal with messages quickly
Inbox has intuitive swipe gestures that allow quickly dealing with emails or groups of emails. It's possible to mark them done, archive them or snooze them for later by swiping. It allows you to keep inbox at zero messages easily.
Pro Surfaces important information from messages
If an email contains pictures, flight information, reservation at a restaurant, attachments, links, dates, phone numbers and other information that the application recognizes, it will surface that information in its main view. These items can be viewed and interacted with without having to open the message itself. It allows finding and taking action on the important email information quickly.
Pro Can add notes to emails
You are able to add notes or reminders to pinned items - very useful for long email chains when you can't remember what the main point was!
Pro Integrates Google Now reminders
For users of Google Now, Inbox integrates with the service allowing for reminders to be seen while using Google Now as well as set through the service.
Pro Integrates realtime updates in messages
Through a feature called Assists, Inbox offers context sensitive updates to messages. These include information such as flight status updates before a scheduled flight.
Pro Can create tasks from dates and tasks in messages
If a message includes a date, Inbox is capable of automatically creating a task from that information.
Pro Auto replies
Can reply to emails using an AI generated response
Pro Trips Bundle
Centralized "folder" for trip info such as airline, hotel, and rental car reservations. Similar to Google Trips.
Pro Categorizes mails by Bundles
In addition to the several automatic categories, you can also create your own bundles. Thus reduces clutter and enables you to archive a whole bunch of mails with one swipe.
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.
Con Only supports Gmail accounts
No exchange email support or importing of any other e-mail services mail.
Con Will be shut down in March 2019
Con No rich-text (HTML) formatting for composing mails
Con No way to mark a message as unread once it's read
Con No way to delete a single message in a bundle
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).