If you find yourself being stymied by connection errors when trying to FTP into your web site's account, AND if you are also hosting your own FTP service on a separate machine/address, you may be simply trying to resolve to the wrong server.

Check the FTP log to verify the IP address of the server accessed when you attempt to connect to ftp.yourdomain.com. If it doesn't match your web site's address, try just FTP'ing into www.yourdomain.com. Your zone file is set up to send ftp.yourdomain.com traffic to your FTP server address, so your web site FTP credentials aren't working.