Out of curiosity do you front-end SMTP with postfix to have many queues/MX entries and a battle hardened front-end or is Stalwart handling inbound connections directly? Im thinking of moving from Dovecot to Stalwart so family members have more modern features on my fallback domains about half of my domains do not use Fastmail. In multiple companies I had several Postfix inbound servers to keep the internet from touching Exchange directly and have multiple nodes for companies to quickly hand off to in multiple locations.