Occasionally new source and target Exchange servers are introduced into the Organization after the Migration project has started, but they do not automatically appear in the Migration Manager Source-Target Pair.
In order to refresh the Project with new Exchange servers that were introduced in the Organization, re-run the Add source organization and Add target organization wizards in the console.
This will re-enumerate the Organization and the new Servers will be available in the Project.
Note: You do not have to remove existing added organizations. Just readd the organizations and the SQL Project database will be updated / refreshed with the information.