Although site-to-site transfers are great for the user, expediting what otherwise could be a slow transfer, many administrators consider site-to-site transfers a security risk, opening servers to an attack known as “port theft” or “FTPing by proxy.” Depending on how you both servers are configured, you may want to block these types of transfers.
Start the Administrator Interface and connect to the Server.
At the bottom of the left pane, click the Server tab.
In the left pane, select the Site you want to modify.
In the right pane, click the Site Options tab.
Select or clear the Block site-to-site transfers check box.
Click Apply.