Mailcow is pretty cool, it's a dockerized stack for running a mail server (with rspamd, webui, etc). Pretty easy to setup and configure. What u was struggling the most with was the spam configuration. A lot of inbound mails were going to spam. Also, big mail providers tend to blacklist mail servers ran by individuals on vps, etc. Check this article out https://blog.lopp.net/death-of-decentralized-email/.