When comparing RedReader vs FairEmail, the Slant community recommends FairEmail for most people. In the question“What are the best Android apps?” FairEmail is ranked 5th while RedReader is ranked 12th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Free with no ads/tracking
The app is completely free and there is no tracking through advertisement since there are none. For those that are security conscious on their devices this makes a great choice for a Reddit app.

Pro Simple navigation
Scrolling and swiping actions make for an intuitive interface that is easy to control.

Pro Open Source
RedReader is completely open source meaning the code is available for anyone to look at or use. It's a completely open environment for understanding the app if one is so inclined.

Pro Clean UI
The user interface is simple in its layout making it easy to read and use.
Pro Truly Ad-free
No ads, open-source.
Pro Open source
Pro Multiple account support
Multiple identities per account, notifications per account or contact.
Pro Lightweight
<6 MB
Pro Fast and customizable
Privacy features are excellent.
Pro Identities support
Pro Two way synchronization
Pro Flat conversation threading
Pro Encryption support
Supports standard OpenPGP encryption for total privacy.
Pro Support for safety and low-bandwidth
Allows to configure maximum size of email downloaded immediately.
Pro Can show plain text of html mails
Cons
Con ugly
Con Does not start gif/video with muted audio
Con No Microsoft exchange server support
Con UI might be a bit confusing for some
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