Is Mattermost available in Spanbox?

Yes, Mattermost is one of the apps Spanbox supports. Add it from the app list, sign in on Mattermost's own page (WhatsApp is a QR scan), and it lands in your unified inbox next to everything else, team channels and DMs. On the free plan it just uses one of your 3 app slots.

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