Check the admin help for Customizing the Web Access application. That should be what you want. Below is the excerpt.
ViewMessage_Format
Controls the display format that is used when the user's browser does not allow cookies or the user has never chosen a format to use. This affects:
Links in custom shortcuts
Items opened from within Archive Explorer
The user can use the Settings dialog to override the behavior you specify with ViewMessage_Format.
WEB (Default) Display the item in a browser window.
MSG Use Outlook to display the item. (Not available on Mac clients.)
ASK Prompt the user for the format to use. (Not available on Mac clients.)