In earlier versions of IIS, the FTP service returns a 125 Data connection already open; transfer starting response for APPE, STOU, and STOR commands sent by FTP clients when the client and server are communicating over a passive mode connection. Additionally, FTP returns a 150 File status okay; about to open data connection. response for the APPE, STOU, and STOR commands over active mode connections.

In FTP 7.5 and later versions, the response message does not depend on whether the request for the data connection is over passive mode or active mode. Instead, if the data connection is already established FTP 7.5 responds with 125 Data connection already open; transfer starting. If the data connection is not already established, FTP responds with 150 File status okay; about to open data connection.

if !strings.HasPrefix(line, StatusFileOK) {
    // MLSD failed, lets try LIST
    if err = ftp.send("LIST %s", path); err != nil {
        return
    }

    if line, err = ftp.receiveNoDiscard(); err != nil {
        return
    }

    if !strings.HasPrefix(line, StatusFileOK) {
        // Really list is not working here
        err = errors.New(line)
        return
    }
}