I'm still confirming this on Outlook, but this is what's happening on the Web Interface:
When I create a calendar event, only some of the attendees free/busy schedule are available. I couldn't figure out what was happening, so I went into the user management screen.
Users that I can see free/busy have a standard person icon beside them, people I can't see look like they have a person sitting in front of a computer icon. (I can get a picture if neccesary).
Also, people I can see show up as:
and people I can't see show up as:
It looks like changing the 'Mail Transport' field fixes this, but I would like to verify this is the right thing to do.