If your newsletter is really interesting and brings important and constructive information, makes a good promotion then it is more likely that more than a few of your contacts forward your newsletter. That is why in all the newsletters that you send, you will see in that in the footer says forward to a friend:
When your contacts are going to resubmit a new page, this will appear:
They will fill this information and the newsletter will be sent to their contacts.
When your contacts click on the forward to a friend button, you will see which one of your clients has sent it, but you will not be able to see the email of the person to whom it was sent.
When this person receives the email forwarded, if it looks interesting enough, the person will most probably to subscribe to your list.
You surely ask yourself why does it not show me the email that was forwarded? The answer is simple, it does not show as there will be people who simply see the web address in their statistics and add to them to their list, and this would not be right since it is not an email opt-in.
So I always go a little further and make your newsletter creative, interesting, innovative, because it may be that more people than you think are receiving it, and can become your recipients, and even be your future clients.