Word of mouth is one of the best ways to grow your email marketing list. Add some encouragement in the actual text of your email, rallying people to share the email.
And be sure to encourage those new folks to subscribe! Add a short call to action to sign up, for any readers that received the campaign via forward.
With the forward to a friend and subscribe links, your list will grow naturally. Word of mouth list growth equals solid, ever improving view and click rates on future mailings.