mailcow - An open source mailserver suite.
Mailwasher - Over 8 million people use MailWasher as their spam filter
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.
SPAMfighter - SPAMfighter and Microsoft have built the strongest and most effective anti spam filter for Outlook, Windows Live Mail, Outlook Express & Thunderbird
Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
Spam Reader - Spam Reader is a free anti-spam add-on for Microsoft Outlook.