It's one tiny binary that does everything you could possibly need for hosting a mail server, including an admin UI, and you get a bunch of modern and convenient features for free.
For example, it automatically handles Let's Encrypt certs for you. You get JMAP, CalDAV, WebDAV, CardDAV, IMAP4rev2, DKIM/SPF/DMARC, MTA-STS, DANE, spam filtering, SQL+blob+object storage backends, search, clustering, OpenTelemetry, etc all in one tiny binary.
Downsides: some features are gated behind an enterprise version and I think the dev team is one guy, or at least it was a while ago.
Having ran both for a long time, I'm sticking with Stalwart from now on as long as development continues.
For example, it automatically handles Let's Encrypt certs for you. You get JMAP, CalDAV, WebDAV, CardDAV, IMAP4rev2, DKIM/SPF/DMARC, MTA-STS, DANE, spam filtering, SQL+blob+object storage backends, search, clustering, OpenTelemetry, etc all in one tiny binary.
Downsides: some features are gated behind an enterprise version and I think the dev team is one guy, or at least it was a while ago.
Having ran both for a long time, I'm sticking with Stalwart from now on as long as development continues.