In order for your program to have access to the network and communicate with a mail server, you need to sign all of the jars and include in the jnlp file this code:
<security>
<all-permissions/>
</security>
If you use the Java EE mail implementation, then you need to include two jar files in your library:
<resources>
<jar href="lib/activation.jar"/>
<jar href="lib/mail.jar"/>
</resources>
I like to place all of the library jar files in a separate folder, which is what netbeans normally does for you when it builds you app (look in the dist folder).
You also asked "can u offer me a way to run this program on my computer without netbeans and command prompt?"
That is pretty much the whole point of jnlp. You can launch the program from a web browser. The java tutorial contains many examples of this:
For example: JButton example
If this is the way you wish to deploy your application, you can read the Web Start developers guide.