Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
Zoho Mail - Zoho Mail is a secure, encrypted, and enterprise-ready email solution, a suite of apps tailor-made for your organization's needs.
mailcow - An open source mailserver suite.
FastMail - Email, calendars, and contacts to highest standards of security, privacy and reliability.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
QQ Mail - QQ Mail is an enterprise email solution from Tencent.