I can recommend Stalwart [1] which is a complete mail service contained in a single binary, that doesn't really have any external dependencies, and is really easy to install and update.

I've looked (and tried) a few other projects in the past, but Stalwart was the easiest to setup, and I haven't had any issues with it so far.

[1] https://github.com/stalwartlabs/stalwart