Stalwart is unique I think. The whole thing was built by essentially one developer in rust, and it's quite amazing how he has done it in just a few years. He's expressed interest in expanding the software beyond email in the past, and contacts/calendar/files shouldn't be too hard of a challenge for him.
That's a bit negative. There are plenty of people that want a full OSS alternative to Gmail, Outlook, Yahoo and others. That includes calendar and contacts.
Imagine maintaining a useful piece of FOSS and then Mozilla shows up and "pushes hard" for some feature they want for a service that's missed the boat by a decade and doesn't even elicit much hope from loyal users (including myself).