I am having this same error "500-Unrecognized command format..." right after the USER commend with other FTP clients (Sorry, not WinSCP.) while connecting to Pure-FTPd server at Bluehost.com. It seems more like a server error.
Turns out this was the result of a firewall blocking the FTP USER command. Not sure why the Pure-FTPd server gave an error response, except maybe it timed out waiting for the USER command that never came. Hopefully this is helpful to someone in the future.