Wide use of modern browsers' capabilities, like drag'n'drop of files, browser notifications, file upload with progress bar.
Rendering messages of any size or complexity. Support for images embedded into messages. Support for multi-language mails.
Full functionality control using keyboard, with keyboard shortcuts supported.
Supports all modern mail servers which provide IMAP and SMTP support. Special IMAP extensions, including QUOTA, CONDSTORE, SORT and MOVE are supported as well.
Supports Sieve scripts (Filters and vacation message)
Multi-level caching system allows for increasing application performance and reducing load on mail and web servers.
Requires PHP 5.3+ and a few extensions you probably have installed already.
Customization of interface layout using visual themes.
Integration with Facebook, Google and Twitter allows users to log in using their social network credentials.
Really simple interface for installing and upgrading allows for setting up RainLoop Webmail even if you're not really technical.
Upgrade tool built into RainLoop Webmail allows to obtain new version or deploy new plugins from admin interface with a single click (Standard edition).
The application doesn't use database backend (except contacts), that makes maintaining and configuring it much simpler.
Allows for working in secure mode with mail servers using SSL and STARTTLS protocols.
Client Side OpenPGP (OpenPGP.js)
Two-factor Authentication (2FA)
Built-in protection system removes hazardous HTML entities for preventing various attacks. In addition, token-based technology is used for protection from CSRF attacks.
Plugin system allows for extending default functionality with no need to modify the code. That allows for keeping custom functionality intact through application code updates.