CalDAV iCal invitation responses not received

I have the iCal Server set up on a Mac Mini server running 10.6.7 and two clients with the CalDAV calendars. The users are in the OD, my account has an active email account with the server domain (me@domain.com), the other has mail forwarding to an external address (them@domain.com forwards to them@somethingelse.com).

If I create an iCal event and set the other user account as an invitee using the them@domain.com address, they get the invite, but their response is never received.

Additionally, if they invite me using me@domain.com, they never get my response on their CalDAV calendar.

And the final odd behaviour is that I can't send responses for invites to my external email account (me@something.com) that I've added to my CalDAV account. Nor can I change the properties of the event without right-clicking and using "Get Info". It just says "Maybe, Decline, or Accept" when I double-click it even after I've selected one of those choices.

Any ideas how I can resolve this?

Thanks