Ranked in these QuestionsQuestion Ranking
Pro Client-side OpenPGP
RainLoop supports an easy to use client-side OpenPGP encryption. It allows setting up, importing and generating keys from the admin panel. OpenPGP keys are stored in the browser's local storage.
Pro Two-factor authentication
Pro No database backend (except for contacts)
Except for contacts (which are optional), RainLoop does not require a database to run, allowing for easier management and configuration.
Pro Social network authentication
Pro Easy to apply your own branding
Not only can you change the title, description and logo with a simple form in the admin panel, you can also completely customize the look of the login screen by supplying your own custom stylesheet.
Pro Configurable multi-level caching system
RainLoop caches messages to allow quicker access to your mailbox and to alleviate the strain put on the server. You can choose to disable it completely if you don't want it. Or, you can configure cache settings such as an additional caching key, browser-level cache, caching message user IDs while searching, etc. Even changing memcache hosts and ports and caching system data is possible.
Pro Keyboard shortcuts
Pro Drag & drop support
RainLoop supports attaching files simply by dragging and dropping them in.
Pro Plugin support
RainLoop has an easy to use plugin system. You can find the full list of plugins in the admin panel (or see their code on GitHub). From the admin panel you can, in a simple, user-friendly way manage them - install, uninstall, update. Unfortunately, the amount of plugins and the related documentation, currently, is somewhat lacking.
Pro Dropbox integration
Pro Easy to update
RainLoop can be updated from the admin panel. It does not require manually downloading, overwriting or configuring anything with each update.
Pro Free for personal and non-profit use
RainLoop is licensed under Creative Commons. Commercial use costs $85/yr.
Con Lack of API documentation
Most "closed" open source ever seen.
Con Questionable start
Con No auto-refresh for emails
Requires to click on a "refresh" button when competition has built in auto-refresh functionality for a long time
Con Small community
No community forums, etc. This makes it difficult to find community help or just discuss techniques.
Con Lack of documentation on plugin development
Con Raises security concerns since it's written in PHP
The language itself and the library ecosystem has a bad security track record.