I'm biased but as far as SMTP is concerned, you can hardly find something simpler than OpenSMTPD: pretty much any configuration can be described in less than 10

I'm biased but as far as SMTP is concerned, you can hardly find something simpler than OpenSMTPD: pretty much any configuration can be described in less than 10 lines of config. Here's my configuration on this desktop: listen on all accept from any for domain debug.poolp.org deliver to maildir accept from local for any relay