[clarry]

Have you tried OpenSMTPD[1]?

I've been running it for a few years now, admittedly with a very basic setup. My config is about ten lines long, written in five minutes or so. I haven't really needed to touch it since.

[1] https://www.opensmtpd.org/

[Torgo]

I love OpenSMTPD. The only issue I had with it was, no filter API (just fixed, as of a month ago or so) and to get things like spam and antivirus filtering, or domainkeys, you have to proxy chain it. Also, the API changed very rapidly, and I had to run nightlies for a while to get around a few issues (I'm on stable now though with no problems.) These things add complexity to the configuration, but I would still rather do this than maintain any other mail server software.

[stevekemp]

I use qpsmtpd as a SMTP proxy, which handles all my anti-spam checks, and does virtual domain lookups.

The setup is documented here, and pretty nice https://github.com/skx/ms-lite/

[thisishugo]

It's on my radar, but to date I've not had (or more accurately made) the time to seriously evaluate it. I'm looking at an architecture overhaul soon, so will make the effort to take a proper look before then.