Does Spanbox support Yahoo Mail?

Yes. Yahoo Mail has a web version, and Spanbox's whole model is that if an app has a web app, you can add it. So you sign into Yahoo Mail inside Spanbox exactly like in a browser and it sits in your unified inbox with the rest, your full inbox. It counts as one of your apps (3 free, unlimited on Pro).

You can even run more than one Yahoo Mail account at once, each in its own isolated session, so a work and a personal Yahoo Mail sit side by side without logging in and out.