I'm a very happy user of Maddy (https://maddy.email/). It's a single executable that contains everything you need for a send-and-receive email server, including all modern anti-spam features. In my experience, the configuration is simpler than Postfix, Exim and OpenSMTPD, and falls back to sane defaults.

Another nice feature is what I would call its architecture: it does not try to be a service manager (instead, you are supposed to spawn it from your own favorite service manager, be that systemd, docker, or anything else, and you can hence really lock it down), hence it does not require root to run and does not rely on using system accounts by default.

I've only used it for very basic setups, but for those I can highly recommend it.