Configuring the DNS4Me Mail Server to Listen Behind a Router/Firewall - KB Article #1405Related Articles --
Starting with version 4.0, DNS4Me contains an integrated mail server. In order to run the DNS4Me mail server behind a firewall, the server must first be enabled and port 25 must be forwarded to the computer DNS4Me is running on.
Examples of setting up a Linksys router are here. If no messages are being sent by the mail server after configuring the port forwarding (you receive non-deliverable message errors for every message you try sending), then chances are that your ISP has blocked the SMTP port to prevent you from hosting your own mail server.