Does Spanbox support email too (Gmail, Outlook, Proton), or is it only chat apps?

Email's very much included, it's a chat AND email hub. Gmail, Outlook and Proton Mail are all supported, so you can have your inboxes sitting right next to WhatsApp, Slack and Telegram in the same window, and their unreads feed the same unified inbox. It's the same mechanism as the chat apps: it loads each email service's web version locally. So the 'half messaging, half email' chaos is exactly the case it's built for, one window for both instead of a mail app plus five chat apps.

Running two Gmails, an Outlook and a Proton alongside my chat apps. Email counts toward your app slots the same as anything else, but having mail and messages in one inbox is the bit that actually declutters the day.