I gave the #Stalwart email server another try. I found a few bugs (quickly dispatched by the team), worked around an issue with my crappy ISP filtering inbound port 25, and got it working.

• In addition to #IMAP4, Stalwart also supports #JMAP, a much-improved spec for clients. Holy cow is that fast!
• I set up an #ElasticSearch index for full text search. Really simple.
• Instead of using RocksDB for storing email, I moved it to a directory backed by an NFS mount. Simple, and now I have effectively unlimited email storage.
• Most stuff you need to do can be accomplished through the web UI. Get a TLS certificate from LetsEncrypt? Just a few clicks. Use DNS for ACME domain authentication? A few more clicks is all you need. Put an extra name on the certificate because your ISP sucks and you need to use an external relay? No problem.
• It gives you all the DNS changes you need to make to set it up, including DMARC, SPF and DKIM, MTA-STS, autoconfig, etc.
• Relaying your outbound email is just a minor tweak.
• Milter support is built-in, so you can bring your rspamd implementation right along.
• Incoming and outgoing TLS & DMARC reports are handled automatically.

I’m sure I’ll find some things that it doesn’t yet do that I’d like, but for now my opinion has changed and I’m really impressed.