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.
It’s also what Thunderbird is using to build their paid email hosting. Seems like a very ambitious project mostly done by a single person – impressive!
Wow! I was just about to comment how email is the one thing where I wish something that didn't follow the unix philosophy existed. Exactly due to this, it is easy to set up a mail server but it is hard to think of all the things around it: spam, fishing, dmarc, dkim, spf, etc.
This looks really nice, especially also for saas projects.
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