The Passive property enables or disables passive file
transfers between the local and server. In passive transfer
mode, the client is responsible for establishing the data connection
between the server and the local system. If the local system
is behind a firewall or NAT router, it is recommended that you set
this property to True.
Note that not all servers support passive file transfers. If the
Passive property is set and the server does not support
passive mode, an error will be returned the next time a file transfer
is attempted.