When you connect to a server that connection is made through what's known as a port, identified by a number. Webservers connect using port 80, mailservers through port 25, and each other type of server has a different standard port so every machine online knows specifically how to connect for any given purpose. Sometimes when you are using a dynamic connection the ISP will be blocking the standard ports for webservers and/or mailservers, usually due to security concerns relating to viruses and worms.

This means that though the domain points to the correct IP, the people looking for your server cannot connect since the port they are trying to use for the connection is effectively closed. Thus to be able to run servers of this kind you will need to use our forwarding services, provided with DNS-plus service, to direct the domain to alternate port numbers.