By default IIS FTP site supports explicit FTPS only. To turn on implicit FTPS, follow these instructions:

1. Open the IIS Manager. Go to Sites > FTP site, and click Bindings.
2. Add a binding with the following properties:
   • Type = ftp
   • IP Address = All Unassigned
   • Port = 990
3. Restart the FTP site and the DefaultAppPool application pool.