I currently use Vimbadmin to manage domains and addresses for my personal email server. So I'd need to be able to keep using it, or be able to replace it for both the new jmap/imap server and postfix. SQL support would let me continue using it.

That said, I think I'll find some time to give Stalwart JMAP a try. I've been curious about JMAP for a while, and there are a couple things about Dovecot I'm not too fond of...

Another vote for SQL support - I use sqlite as my user backend for Dovecot. Definitely interested in giving Stalwart a try though.