This KB article title is a bit of a misnomer, but once you dig info this you’ll see it’s not just the fact that your mail is from exchange, but more related to a MIME to MAPI conversion problem. This KB relates to properties which begin with X- (it’s something that Microsoft IT uses when they retire or replace something). If you’re experiencing this issue it may related to MIME to MAPI conversion issues.
This problem occurs because Exchange 2003 cannot convert the MIME-type properties to MAPI-type properties correctly if the names of the properties begin with X-.
You cannot open a SharePoint Server 2007 notification message in Outlook 2007 when your mailbox is on an Exchange 2003 server
The hotfix is available via customer support. The hotfix applies to Exchange Server 2003 and has to be applied after it has been upgraded to SP2.