iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Litmus - litmus tests and tracks your email campaigns, so you can always put your best design forward.
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.
Brevo - Innovative online Email Marketing solution to manage your contacts, create & send your newsletters and track your results. More than 80 000 clients. Best prices and attractive features.
mailcow - An open source mailserver suite.
Email on Acid - We make the send button safer. For every campaign, every message, and every inbox, Email on Acid has the time-saving tools you need to put your best email forward.