Request for help: External invites arrive in IMAP but not in calendar

[wpk-]

I have Sabre/DAV set up and working. The groupwareserver.php example works very well. I've set it up with IMAP authentication and that works, too. When I add a new event and invite externals, the invitation is sent and arrives in their calendars.

But when an external organiser invites me, the invitation only shows up in my email and not in my calendar. When I accept the invitation, the organiser sees that, but still the event does not show up in my calendar.

The main issue is I don't understand how this works. When an external invites me, does their server try to connect to my webdav server or is it up to my webdav server to poll my imap server for new invites? If I understand that, I hope I can find a solution more easily.

Help greatly appreciated! Thanks