I just figured this out last week. The idea popped into my head that there must be a way to send my newsletter to recipients who never opened it the first time I sent it out. If they did not want to hear from me, they could always unsubscribe, I figured.
This is only if you use Mailchimp. I am sure there is a way to do it in the other newsletter services out there, but here it is in Mailchimp.
It is a bit tricky but I think it is a good way to get the word out and not look like you are spamming people over and over.