Forwarding by filtering doesn't work

Currently I only purchase domaindomain.nl from a registrar. Mail intended for info@domain.nl is delivered to an external server (1.2.3.4). On that server runs cpanel and through e-mail filtering I try to forward mail from a certain sender to an external e-mail address. However, this does not work: although 'from' does specify the correct e-mail address, the registrar's server ip address is listed as the actual sender, which means that the filter set in the cpanel does not recognized the e-mails from this certain sender. How can I solve this problem?