open source mail server
Mail-in-a-Box
iRedMail
Modoboa
Mail Transfer Agents
https://www.proofpoint.com/us/open-source-email-solution
Mail Delivery Agents
Reference
https://www.linuxbabe.com/mail-server/ubuntu-18-04-iredmail-email-server
https://www.linuxbabe.com/mail-server/email-server-ubuntu-18-04-modoboa
https://www.linuxbabe.com/mail-server/ubuntu-16-04-iredmail-server-installation