Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Gmail - Gmail is available across all your devices Android, iOS, and desktop devices. Sort, collaborate or call a friend without leaving your inbox.
Roundcube - Web-based IMAP email client
Yahoo Mail - Web-mail with a terabyte of free storage space & spam protection. Yahoo!
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.