Calendar: Accepting an event launches Mail. Why?

I'm experiencing a very, very annoying feature/bug in Calendar or Mail. Whenever someone sends me an event invitation via email (as an .ics file), and I accept it, Mail launches. Mail is not my default email app, and I don't use it. I'm using a third-party app called Postbox, but it shouldn't matter since we're dealing with standard .ics files.

So my question is simple. How can I add or modify an event to Calendar without Mail launching? In my case, it's extra frustrating because Mail will hang and stop responding at least 50% of the time. Also, I never configured the email addresses so it only checks my @me.com account.

The whole thing is baffling and frustrating. I appreciate any insight on how to decouple the two apps. Does this have to do with notifications and the always-on nature of mail?