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 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 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 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 Two-factor authentication
Pro Free for personal and non-profit use
RainLoop is licensed under Creative Commons. Commercial use costs $85/yr.
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 RainLoop plus Xeams
Complimented modern user interface to deliver mail from Xeams. Setting up both applications takes less than an hour on private server.
Pro Drag & drop support
RainLoop supports attaching files simply by dragging and dropping them in.
Pro Keyboard shortcuts
Pro Social network authentication
RainLoop integrates with Facebook, Google, and Twitter for authentication. You can find out how to set up each of the services here.
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, a number of plugins and the related documentation, currently, is somewhat lacking.
Pro Dropbox integration
RanLoop allows attaching files from Dropbox. To enable this feature, go to Dropbox's developer apps page to register your app. You'll receive the app key value to enter in the admin panel.
Con Lack of API documentation
Most "closed" open source ever seen.
Con Lack of documentation on plugin development
RainLoop sends the user to a plugin example that is nonexistent (which they state will be filled in later), which means the user has to figure out what the process is on their own.
Con Small community
No community forums, etc. This makes it difficult to find community help or just discuss techniques.
Con No unified account view
Con Questionable start
The developer created RainLoop while working at AfterLogic. You can read more about it here.
Con Raises security concerns since it's written in PHP
The language itself and the library ecosystem has a bad security track record.