As someone who uses and loves Caddy, I'm very excited about a mail server with a similar philosophy. I'm rooting for you!

That said, Caddy's killer feature for me was automatically configuring certs, that's what made me switch from Apache back when we were moving everything to HTTPS. I still don't fully understand how certs work, but fortunately I don't really need to. Until Maddy does this, it won't be a good comparison.

Also I would really appreciate some documentation for making this work with Caddy handling TLS certificates for me. I guess I'll file a bug about that.