Mox: Modern, secure, all-in-one mailserver
xmox.nlHow does its IMAP4 fare with ImapTest?
I've not run it against it yet, but it's part of their Makefile: https://github.com/mjl-/mox/blob/2c003991bb98025582195d4f6a4...
If this had been around 20 years ago, I suspect hosting your own mailserver wouldn't have fallen out of vogue.