Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
mailcow - An open source mailserver suite.
MailClark - The Slack bot for external communications
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.