Cannot access IIS FTP server from outside ( error 426 )

was wondering if anyone could help me out. Im trying to set up a corporate ftp server. Im able to connect no problem through the servers local IP address. Whenever I try to log on from the outside i get the login window but then when i log in i get a message saying ther was an error logging on with the following details :
200 Type set to A
227 Entering passive mode
426 Connection Closed; Transfer Aborted

Im running a mail server on the same machine and have no problems connecting from the outside. ANY help or suggestions would be appreciated.