When comparing Aqua Mail 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 Aqua Mail is ranked 4th.
Ranked in these QuestionsQuestion Ranking
Pro Top rated for keeping your info private
Has been rated well on AndroidForums.
Pro Ultra fast and intuitive
The app refreshes emails in seconds and has an intuitive menu. You can do a lot with one or two swipes.
Pro Integrates with cloud storage services for backup and restore
It's possible to backup the app along with account settings via services like Dropbox, OneDrive, Box.com, Google Drive.
Pro Integrates with Tasker
Tasker is an automation app. Aqua Mail's Tasker integration allows changing settings, triggering message based events and running mail checks based on criteria like time of day, location, timer, connection type, etc.
Pro Pro version often goes on sale
The developer usually gives a 25%-50% discount every few months or so on Google Play.
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 Floating ads in free version
It shows floating ads in the email list and they can not be deleted.
Con Limited signature capabilities
Signatures can't have images or HTML.
Con Does not support pgp or metacert
Con Free version supports only 2 accounts and adds promotional email signature
The base, free application supports up to two accounts and adds a promo signature to all outgoing messages.
Con No support for Exchange Activesync protocol
This is the de facto, secure Exchange over the web protocol used and is enforced by most businesses for enterprise email. Aqua only supports EWS for Exchange, which most enterprise administrators block or disable.
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).