iCal doesn't send email to invitee...

Greetings,

We have several computers/iPads/iPhones at home using IMAP mail service from our own domain. We currently share the addressbook between my wife and I, and have 4 email addresses between us having one main address card.

Since we share the addressbook and Apple ID, we also share the same iCloud calendar. For some reason, when I send a calendar invite, the invite never receives email notification and I have noticed that there is a grey question mark by the user.

Not really sure what this means but all data is basically in the cloud as we don't utilize any local calendars anymore. (This computer)

Cheers