Interesting Email Behavior

I know that Pivotal Tracker has always had some issues regarding the delivery of email. At my latest employer, I created a project and invited a number of co-workers. Not a single person got the Project Invitation email. In trying to figure out how they could still access the project, I had them reset their password, which of course sends a Password Reset email. Surprisingly, everybody received the Password Reset email. Continued use of Tracker revealed that everyone was able to receive the Project Story email notifications as well.

Makes me wonder what makes the Project Invitation Email "special".....