I found this on their Github page. Mailspring is basically hardwired to use libsecret on Linux, and libsecret doesn't work with kwallet.
To get it working it appears you'll have to install gnome-keyring, and add it to your startup applications. The syntax for the startup is: gnome-keyring-daemon -s

That works! So... this should be in the Mailspring dependencies then?

I would think so. Seeing how all the other Solus desktops are GTK based, they have gnome-keyring by default. Plasma doesn't, so I suppose they'll have to add some dependencies to quite a few packages now to ensure they'll run under Plasma.

I'm actually a bit surprised it's not in the snap though.

So we can add gnome-keyring to our build, but it won't help people using the snap.

Also won't be done until it builds and works again.

Well, I'll use Thunderbird in the meantime. It's true that Mailspring is not working properly, and I consider installing gnome-keyring a non desirable solution if you want to use Plasma.