Error Message: “not supported calendar message.ics” in Outlook 2007 customers

From a program I make a calendar arrangement detail (ics file) and throw it to email receiver. This has been running very well with Outlook 2003 clients on the other hand Outlook 2007 clients are facing difficulty. They obtain it as an attachment with the heading:
Not supported calendar message.ics
Is there an answer to this like for SP or hotfix?

