In case the FTP PASV command – the FTP protocol command for the passive mode – is casted behind the firewall, the proxy server, or the DSL router, the FTP server will more likely return an address different from the computer casted the command. The returned address is the IP facing the outside world. It is normal and very common. Most FTP clients, even Microsoft very own FTP client ftp.exe, are ok with it. Too bad, the .Net class FtpWebRequest will report the following error. And, they say it is for the security reason.
“The server returned an address in response to the PASV command that is different than the address to which the FTP connection was made”
The problem is expected to be fixed in NETFX 4.0.