Interesting, nice they use this rust, I am curious how they define this scaling. What I don't get is why are they messing with smtp. I always get a bad feeling when a company is trying to do everything.
Good they are using rust and even better they've had an independent security audit (https://www.stalw.art/blog/security-audit).
On the scaling side, maybe see the storage page ? (https://www.stalw.art/docs/storage/overview). The metadata is stored in a database which can be replicated. And the mails themselves can be stored in filesystem or "S3-compatible" storage, and so there are scaling options there too ? But clearly some experimentation is required to see how it works in practice.
Are they messing with SMTP ? As I understand it its an IMAP/JMAP server. And (like Dovecot) it has LMTP for getting mail into it from e.g. Postfix ? From my reading of the docs it looks like SMTP is only there if you don't want to use LMTP to get mail into it ?