How can I determine which named property (or properties) are filling my exchange 2007 information store?

It isn't a matter of a single named property causing this, rather a profusion of them. I've had this exact problem several times in the past couple years. In the beginning, Microsoft saw fit to allow any old X header in mail to promote to a named property in Exchange. They also saw fit to hard code the maximum number of named properties. Time passes, and jokers do this kind of thing in headers:

 X-IHateYou: True

Or other software packages doing stupid stuff like:

 X-20010122113201: True

Which Outlook dutifully promoted to a NamedProperty. And people started running out of NamedProperties quota. Microsoft has put out a few articles on this over the years.

A few resources:

• Named Properties, X-Headers, and You
• Named Properties, Round 2: What lies Ahead
• Increasing the Named Properties Quota

What you are seeing is not one NamedProperty getting used to exhaustion, you actually WANT those to go to NamedProperties. What you don't want are X-headers that you don't give a wet noodle for getting promoted if it doesn't have to. Exchange 2007 has had some progress on this issue in recent service-packs and update rollups as the linked articles suggest. However, if you're running into it now, the only thing left is to increase your NamedProperties quota.