If Postbox is set as your default mail client on Mac OS X Snow Leopard or Lion, Apple's iCal will use Postbox for sending calendar notifications.

PLEASE NOTE THAT IN MOUNTAIN LION, APPLE REMOVED THE APPLESCRIPT DIRECTORY THAT POSTBOX NEEDS TO COMMUNICATE WITH ICAL. THE DEVELOPERS ARE INVESTIGATING POTENTIAL SOLUTIONS, BUT WE HAVE NOTHING TO ANNOUNCE AT THIS TIME.

IMPORTANT NOTE: Versions of Postbox purchased through the Mac App Store do not contain support for iCal in order to comply with Apple's Mac App Store Guidelines. Thus, you can:

1. Continue to use the version of Postbox managed by the Mac App Store, without iCal integration, or
2. Download and install Postbox from the Postbox website, for free, to enable iCal integration, however, this version will no longer be managed by the Mac App Store, and updates will be provided by Postbox.

Setting Postbox as Default

• Make sure that all email clients are closed
• Go to Apple Mail / Preferences / General and select Postbox as the default email client.  
• Close Apple Mail, then start Postbox.

Notifications

Meeting notifications will be sent in the background, with a copy of the notification placed in the Sent folder. Or, if you right-click on a meeting and select Mail Event within the contextual menu, the event will be attached to a new message window in Postbox.

Trouble Shooting

In order to make iCal use Postbox, Postbox needs to update an AppleScript file in the iCal application bundle. If iCal still opens Mail.app after you make Postbox your default mail client, make sure you have write access to iCal on your system.

• Quit Postbox and iCal.
• 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 & 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." 
• Start Postbox.

Restoring iCal Settings

When Postbox starts up and discovers it is no longer the default mail client, it will restore the original script file in iCal to open Mail.app. To revert your default email client back to Apple Mail, select Preferences from Apple Mail's Mail menu, click on the General tab, and select Mail as your default email reader. Now open Postbox one time and quit. iCal should now open Mail.app for calendar notifications.

Disable iCal Integration

To prevent Postbox from modifying the iCal bundle when it is the default application, go to Preferences / Advanced / General / Config Editor. Enter: pb.osx_scripting.iCal into the text box and then toggle this setting to false.