Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
FastMail - Email, calendars, and contacts to highest standards of security, privacy and reliability.
mailcow - An open source mailserver suite.
QQ Mail - QQ Mail is an enterprise email solution from Tencent.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.