Many ISPs (companies providing users a connection to the Internet) block outgoing traffic on port 25. This makes it impossible for hMailServer to connect directly to other SMTP servers on the Internet. A workaround for this is to route all your outgoing email through your ISP's mail server.
Also, for different users, people may want to route all outgoing email through a 3'rd party service such as gmail.
The above instructions are valid for 3.4 and later.