Horde Alternatives

Web-based IMAP email client

A fully fledged, free email server solution, an open source project (GPL v2).

RainLoop is a web based email client.

Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.

SOGo is groupware server with a focus on scalability and open standards.

SquirrelMail is a standards-based webmail package written in PHP.

open-source

Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more.

An open source mailserver suite.

All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.

Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.

Kolab Groupware is a secure, scalable, reliable, flexible, full-featured collaboration suite.

Gmail is a free search-based web-mail service.