Accept or decline using more than one email address in Apple Calendar

I Use both Mail and Calendar from Apple. My primaire e-mail adres is connected to my iCloud account. That means that I can accept and decline invites in Calendar for that e-mail adres. But for other mail addresses I can't.

Apparently my email provider doesn't offer CalDAV. Does that mean that I can't sync my other E-mail adressen with Apple Calendar? And use the 'accept' and 'decline' options?