Create C # invitation for Blackberry

I need to create an email in C # that will be sent to BlackBerry (or Outlook) and will be recognized as an invitation. At the moment I am creating an email with the attachment text.ics included so that in Outlook the user can click on the attachment and add it to their calendar which is working. The problem is it is not recognized as an invitation in Outlook to invite email or to blackberry.

thank