NeverBounce is the one I'm most familiar with, but I think a good question to ask is whether you want to do this on an ongoing basis or not. NeverBounce, for instance, has a number of different integrations (https://neverbounce.com/integrations/) so that you're cleaning your database on a continuous basis.
You can even catch bad addresses at the point of entry with these services, so that if the email address entered into a form is well formed but still incorrect, the service can flag that and notify the user before it's even submitted.
My take? Focus on offering the right sharing options on the pages you're driving users to and embedding some sharing options for those pages right in the e-mail.
Not as natural as forwarding, but gives you much more control over the content that's being shared.
Thanks, Jaina! Yes, I've been using the subject line checker, and I think it's great. The one thing I don't like about it though is that it's only available through Checklist.
I'd like to see that tool "out front" somewhere, maybe as another item in the left bar, so that copywriters could do the early work of testing their subject lines and preview text before they send me their copy. (I don't write copy for our campaigns.)
Similarly, for the copy itself, I'd like to see some tool, similar to the Hemingway App, which would help them apply best practices before they send copy on to me put together in our templates and tested.
I've submitted my original comment to the firstname.lastname@example.org address. : )