When connecting to the server via ftp and you are sure about the credentials and the firewall rules and iptables. Check DNS and the proftpd.conf file:
# dig domain.com domain.com. 21599 IN A 216.xx.xx.xx
# nano /etc/proftpd.conf Port 21 MasqueradeAddress 216.xx.xx.xx PassivePorts 60000 65000
Check the MasqueradeAddress feature and see if matched your IP for the domain you are connecting to.
Also – if behind a firewall…
Create a VirtualHost for 216.xx.xx.xx with the following configuration:
<VirtualHost 192.168.0.5> ServerName "ProFTPD" MasqueradeAddress 216.xx.xx.xx PassivePorts 60000 65000 </VirtualHost>