T-Online.de is a German news portal and ISP which provides an email service with address books and calendars.
Password: your password for the given username
With free (non-paid) t-online.de accounts it's possible that only address book synchronization is available (no calendars). Address book and calendars are available for t-online.de customers.
You can mount folders from the MagentaCloud using the DAVx⁵ WebDAV mounts feature.
User name: see CalDAV/CardDAV
Password: you have to set up a WebDAV password