Because a link to an external domain is really just a way of defining a link between a domain on your local GroupWise system and a domain on another GroupWise system, the flow of messages is identical to the flow of messages between any two GroupWise domains. For more information about message flows between domains, choose the type of link that you want to know more about in Message Delivery to a Different Domain
in the GroupWise 8 Troubleshooting 3: Message Flow and Directory Structure.