Maybe this is just me but this project leans quite heavily in MailCow's world. At least from a design standpoint.
Interesting project
Mailcow bundles 3rd party applications (Dovecot/Postix) into a single easy to use solution. WildDuck is a standalone application that handles mail store, IMAP an POP3 all by itself. This also makes it possible to add features like automatic GPG encryption of messages uploaded via IMAP which would be difficult when using off the shelf IMAP server.
IMAP/POP3 - not SMTP
"You can bundle WildDuck with Haraka and ZoneMTA to create a complete mail server." from https://wildduck.email/
Looks like it leans on LMTP for incomming email. It's a little unclear if it does any sending and/or forwarding.
WildDuck has dedicated plugins for Haraka and ZoneMta. So when Haraka receives a new email the recipient is validated by WildDuck plugin and the final message is stored directly to WildDuck’s database.