How do I stop iCal from launching Mail.app?
The instructions below (posted here) did it for us as our permissions were, actually, on "Read Only" and the Mail.app was still launching. 

Note: If iCal still opens Mail.app for mailing an event after you have set Postbox as your default mail client:


Within the Finder, right-click on the iCal icon and then select Show Package Contents.
Navigate to the "Contents" folder, then the "Resources" folder. Right-click on the folder called "Scripts" and select Get Info.
Expand the Sharing &amp; Permissions box and click the Lock Icon in the lower right hand corner to unlock this setting. Change any permissions that are listed as "Read Only" to "Read &amp; Write."
Restart Postbox.


If more information is needed, check out a more detailed Postbox iCal Integration page for more options and troubleshooting tips.

Have a great day!