ETRN and firewall conflict

We have SBS 4.5 and running proxy and Exchange over a dialup with a fixed IP address. It works fine without the firewall, but as soon as we put up the proxy firewall with the appropriate ports open we cannot receive e-mail even though we manually request it from the Exchange management program. Is there a port that the ETRN request needs in addition to the normal SMTP ports?

Thanks in advance for any help.