Outlook Meeting Requests, what are the limitations?

We are always using the invite attendee feature with outlook, but users sometime find that they will receieve the notification as an email and there is no button to accept, decline or tentative and it will not update calender. Is there a known problem with outlook? The other frustrating thing is that it does it spermatically with all users, it is almost like a lottery on whether it will work or not and I have not been able to see a pattern on when it doesn’t work.

We do use a mixture of outlook 2000,2003 and 2007 clients we are on exchange 2007. We sometimes invite people outside of the domain but I have not had problems with that before in my experience and my personal email can send invites to the domain