Here’s the Deal

K-9 vs Nine - Exchange ActiveSync

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When comparing K-9 vs Nine - Exchange ActiveSync, 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 Nine - Exchange ActiveSync is ranked 8th. 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

Exchange SupportWebDAV only (pre 2017)

File SizeVaries with device

IMAPYes

In App PurchasesNone

Lock AppNo

Multi-AccountYes

PlatformsAndroid

PublisherK-9 Dog Walkers

Push NotificationsYes

Required Android VersionVaries with device

ThreadedYes

Unified InboxYes

FreeNo

Open SourceNo

Ranked in these QuestionsQuestion Ranking

Common Questions

What are the best e-mail clients for Android?

Pros

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.

Pro
Really fast

Pro
Great UI

The stack approach is one of this apps strongest points, as it provides the fastest access to mail and is the cleanest to sort through.

Pro
Incredible feature set

Calendar, contacts, notes, tasks.

Pro
Many widgets

Pro
Very reliable

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Cons

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).