I really like where Stalwart is going, but I am quite hesitant to use it when basically all commits are authored by a single person. What would happen if he abandons the project or disappears?
Why not? If it goes away, it would take years until it rots and becomes unusable. And given that it uses open standard, it would be pretty simple to take a backup (which you should be doing regularly anyways) and move somewhere else.