Sometimes you might need to change the port on which your virtual SMTP server is listening to or add additional port(s) on which the virtual SMTP server accepts messages. For example your ISP might be blocking the default port 25 or you might have specific applications which send messages to a different port.
Here you can see how this can be done. It is interesting to point out that you do not need an additional IP address in order to add an additional port for the SMTP server.