Mail-in-a-Box lets you become your own mail service provider in a few easy steps. It's sort of like making your own Gmail, but one you control from top to bottom. Technically, Mail-in-a-Box turns a fresh cloud computer into a working mail server. But you don't need to be a technology expert to set it up. More: https://mailinabox.email/
Mailcow is a slightly more advanced mail server perfect for those with a bit more Linux experience. It has everything you need in a Docker container: A mailserver with DKIM support, antivirus and spam monitoring, webmail and ActiveSync with SOGo, and web-based administration with 2FA support. More: Mailcow Dockerized docs