From: DeskNow Knowledge Base
http://www.desknow.com/kb/

How can I receive mail without a fixed IP address, or without using port 25?
http://www.desknow.com/kb/idx/4/137/article/

We recommend using the DynDNS.com service ( http://www.dyndns.com ).

1) setup a name like mycompany.dnsalias.com at http://www.dyndns.com/services/dns/dyndns/

2) get a MailHop forwarding service. DynDNS will receive mail for your domain, and then forward it to your server mycompany.dnsalias.com, on a port number that you choose http://www.dyndns.com/services/mailhop/relay.html . Please note that MailHop is a paid service.