The MTA handles message transfer between domains.
The message flow diagrams in GroupWise 8 Troubleshooting 3: Message Flow and Directory Structure focus on TCP/IP links because they are the most common and convenient (unless you have two domains on the same server). For diagrams that include mapped/UNC links, see GroupWise 6.5 Troubleshooting 3: Message Flow and Directory Structure. For an explanation of link types and link protocols, see Understanding Link Configuration
in Domains
in the GroupWise 8 Administration Guide.