Can I use Spanbox for just email (Gmail, Outlook, Proton) without any chat apps?

Absolutely. Plenty of people run Spanbox purely as a unified mail client, add Gmail, Outlook and Proton Mail (up to 3 on free) and skip the chat apps entirely. All your inboxes in one window with a single unread list. You're never required to add any messaging apps.

Two Gmails, an Outlook and a Proton in one window with one unread list is a genuinely nice way to run email, even without a single chat app.