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:

1. Within the Finder, right-click on the iCal icon and then select Show Package Contents.
2. Navigate to the "Contents" folder, then the "Resources" folder. Right-click on the folder called "Scripts" and select Get Info.
3. Expand the Sharing & 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 & Write."
4. 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!

The non useful way is not to set email as the reminder action in iCal.

More constructively is to write an Automator action that uses Postbox and then use that automator action as the reminder action.

I have an automator as an app and this is entered in the iCal GetInfo as

To create an iCal Alarm - Start automator and the first thing offers options for the workflow and one of them is an iCal Alarm - The automator then can include a shell or applescript script to call your mailer

I had the same thing and found the answer on SuperUser:

Therefore, unless you disable Mail.app responding to this AppleScript call (tell application "Mail"), there's really only one way to kill Mail.app (as per my comment on waiwai's answer):

sudo chmod 000 /Applications/Mail.app/Contents/MacOS/Mail

To reverse:

sudo chmod 755 /Applications/Mail.app/Contents/MacOS/Mail