mailcow - An open source mailserver suite.
Spamihilator - The free Anti-Spam-Filter Spamihilator removes more than 98% of spam mails before they even get into your inbox.
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.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
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.