The GMX Organizer can be synchronized with any calendar applications that support the CalDAV standard. GMX offers this option as an additional service and has no influence on whether and at what price applications with CalDAV are available for your device or system.
You can transfer appointments from a calendar application to the GMX Organizer if the calendar application supports exports in the "iCalendar" (.ics) format.
|Calendar application||Synchronization with CalDAV||Exports in the iCalendar format|
|Thunderbird||Yes, CalDAV address:
https://calendar.gmx.com/begenda/dav/(your GMX e-mail address)/calendar
|Mac OS calendar||Yes, CalDAV address: https://caldav.gmx.com/||Yes|