Hi James,
I don't think you have anything to worry about here.
I'm assuming you publish the content on your WordPress site first, before you send out the web-based email? If that's the case, Google is pretty good at identifying the original publisher of content and devaluing other web pages that feature the same content.
If you are worried about it and can easily add "noindex" tags to these web emails, do it. If you can't "noindex" them, I wouldn't be concerned about any duplicate content "penalties".
Cheers,
David