Are you trying to connect to an FTP server, likely using filezilla and coming across:
Response: 500 Invalid PORT Command.
Error: Failed to retrieve directory listing

*SOLUTION*
1. Check to make sure Active or Passive mode are enabled depending on which your server uses. If you're on Windows server you likely use Active mode.
2. Check your firewall / router to ensure port 21 is forwarded to your computer or is open in general to receive connection.
3. Log into your remote server, or contact your host to do so, and make sure the FTP client you are using allows a connection from your external IP address.

To allow access to your IP in Windows Server go to your Internet Information System 6.0
1. Right click FTP Server you are trying to change
2. Click Directory Security tab
3. Change button to Denied access so it reads: By defalt all computers will be Denied Access, Except those listed below.
4. Click Add...
5. Add your IP address found at http://www.whatismyip.org/

Enjoy!