"As the meeting organizer, you do not need to respond to the meeti

I have a user that is having an issue dragging meeting requests from an
attachment in his email to his calendar. First, the attachment icon is
different from those that work successfully. Successful attachments have a
calendar icon, while the ones that give him the message of "As the meeting
organizer, you do not need to respond to the meeting" error message have a
yellow envelope in the top, right corner of the calendar icon that resembles
the envelope that shows up on your system tray when receiving a new email.

He is the only user in the domain currently having this issue so I believe
the exchange server can be ruled out.

The problem occurs when both users are using Outlook 2007 inside and outside
of the company.

I've found very similar issues in Microsoft Support Center, but the issues
don't quite match (i.e. a similar error message is said to be caused by
receiving from Lotus Notes users, etc.)

Any help would be greatly appreciated.

Thank you.