RainLoop is a web-based email client designed to provide a fast, simple, and modern interface for managing emails. It supports IMAP and SMTP protocols, allowing users to work with various email services such as Gmail, Yahoo, and Outlook. RainLoop is known for its lightweight design, seamless installation, and user-friendly experience, offering features like address book integration and support for multiple accounts. It is available for installation on various web servers, making it a popular choice for users seeking a customizable, open-source email client.