I'm going to try and convince you not to use a XenForo plugin for this, for the same reasons the email newsletter I'm working on isn't going to use the XenForo forum software either.
First, you'll want to keep your email newsletter traffic separated from forum notifications and the other lower quality and/or transactional emails generated by your forum and other sites that you host.
I'd confirm addresses a single time using the email newsletter service. Once the member opts in, you're good as long as they show activity (reading, clicking, etc) and don't ignore the email. Starting with the "Allow email from admin" or whatever you are doing to identify them in XenForo, send that full list to the newsletter confirmation form to make sure the users want to start receiving the email newsletter. Include the directions to uncheck the appropriate box in their forum settings too. Start everything unchecked. At different intervals than your newsletter, then you can start sending the emails from the last 30 days through the same process. Have your fun sending great email newsletters with useful content that your subscribers love to receive!
The newsletter service will handle unsubscribes. The newsletter service likely has a much better chance of making your subscriber inboxes than your forum server. Keep your complaint rate extremely low and you should be good.
You don't have a lot of subscribers yet. This should be free or very cheap depending on the service that you choose. I'd start by checking out
AWeber and
MailChimp.
Maybe
@VinnyOHare has some additional suggestions.