We have one user who doesn't receive any emails with .ics files attached (external calendar invites). According to Exchange the emails are delivered however they never show up in the Inbox, or Junkmail (and are not blocked on the spam filter).
We are running Exchange 2007.
The user has a MacBook Pro running Mail (OS X Lion), an iPad and iPhone. (All have the Exchange account and none receive external invitations).
Everyone else in the organization is able to receive these emails.