The reason for " 503 Login Authentication Failed" is incorrect ftp login information. So you should always make sure the following details are correct.
1. Host = domainname.com OR IP address (If you are using domain name, confirm that the domain name is resolving to the correct IP address)
2. Username = firstname.lastname@example.org
(no need of @domainname.com if its the root user for the account)
3. Password = your password that you use to access the settings for the specific site/user account.
Also confirm FTP server use port number 21 to transfer files.