A firewall exists between the Migration Manager Console, and Exchange Agents.
The Following ports are used by QMM Exch:
135 and 137-139 (SMB ports, required for logon and file copies)
389 from the source agent host to the Quest console (ADAM project)
389 from the source agent host to the target DC specified in the console (DC update at switch)
1024 - 65535 - (RPC for MAPI )
For more information review SOL39743:
Additionally the agents working in source environment will connect to target agent hosts and to target Exchange servers, in order for this to work name resolution should be working and target computers should be accessible.
For steps to configure the tool to use specific (preferred) DC and GC for mail switching please see the KB article SOL24219: