Recs.
Updated
Specs
Pros
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 Dropbox integration
RainLoop 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.
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.
Cons
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 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.
Recommendations
Comments
Flagged Pros + Cons
Pro Community version is available
There are 2 different versions of Rainloop. A "community" version is licensed under AGPLv3 allowing it to be integrated with all open source software, while a "standard" version is licensed under the "RainLoop Software License" which is defined here.
Con Cannot setup email address different than login name
Default email address is always <username>@<domain> and it seems impossible the change it to <firstname>.<lastname>@<domain>
Out of Date Pros + Cons
Con No auto-refresh for emails (Not anymore)
Requires to click on a "refresh" button when competition has built in auto-refresh functionality for a long time. (That's not true anymore)