Cannot FTP even if ports 20 and 21 are open

There may be cases where you close off all ports and leave ports 20 and 21 open, but you are still unable to FTP. This is because FTP is a complicated operation. For instance, it can also use port 53 for DNS resolution. For cases like these, consult your office system administrator so that he/she can isolate which ports you need to open for your FTP to work.