When comparing Mailbox 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 Mailbox is ranked 22nd. 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
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Pros
Pro Great features for processing mail quickly
Mailbox brings simple to use, swipe gesture-based ways of dealing with your incoming messages:
- A short swipe to the right archives a message
- A long swipe to the right deletes a message
- A short swipe to the left snoozes a message (puts it off until later)
- A long swipe to the left adds a message to a list (folder)
Pro Allows putting off messages until later
Swiping an e-mail message from the inbox to the left gives an option to 'snooze' it for a set period of time. You can manually enter the date for when to return it to the inbox or choose from options like 'later today', 'this weekend', and 'someday'.
Pro Message composer has a dedicated camera button
When composing a message, a button that allows for either taking a photo or quickly adding one from your library is at the bottom-right corner.
Pro Automates common actions
This app via a feature called 'auto-swipe' tries learning behavioral patterns and take appropriate actions on its own, like organizing mail into folders, snoozing certain mail, etc. You can view set rules and manually add and change them according to your needs in an easy to understand way.
Pro Free
Mailbox is available for free, although a premium version with added features might be in the works.
Pro Good looking design
Mailbox has a minimalist design. It divides your incoming messages in three categories - your inbox, things you've postponed and things you've gotten rid of.
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 Can't save drafts
Con No access to Gmail drafts folder
Con No Gmail labels
Con To be discontinued
Mailbox will be shutting down on February 26, 2016.
Con Privacy concerns
There's concern of privacy, because Mailbox has access to your emails, since they're passing through their servers. After being acquired by Dropbox these concerns have been somewhat alleviated, because of Dropbox's stable track record, but not completely.
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.