Can I add Zoho Mail to Spanbox?

Yes. Zoho 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 Zoho Mail inside Spanbox exactly like in a browser and it sits in your unified inbox with the rest, your inbox and folders. It counts as one of your apps (3 free, unlimited on Pro).

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