Keep an eye on OpenSMTPD. For a taste, look at the sample configurations at https://calomel.org/opensmtpd.html.

[Warnings: OpenSMTPD apparently hasn't eaten anyone's mail yet, but it's hardly done. By design, it doesn't include as many options as other mail servers, some of which may be useful. I don't think OpenSMTPD been ported to non-OpenBSD yet. calomel.org tends to be out of date, inaccurate or dangerously incomplete.]