The company recently upgraded to Exchange 2007. Since the upgrade users who are still on Outlook 2003 (SP3) get the following error for some meeting requests:

You cannot respond to a meeting without an organizer. You must add an Organizer field to the item.

The item has an organizer field filled in.

• Exchange is 2007 with SP1
• MS Office (incl Outlook) is 2003 with SP3
• Some users are using iPhones to accept / reject meetings. The iPhones never get an error like this.
• There are no delegates for the user attempting to accept or reject the meeting.
• Sometimes the meeting was previously accepted, sometimes not. We have not detected a pattern in the meeting requests that trigger the issue.

I've found a few people asking the same question on other websites but no answers. Experts Exchange claims to have an answer but from looking at the discussion at the bottom of the page I'm pretty sure they don't.