I disagree with your analysis.
The person who should be subject to the defamation claim is the person that wrote the original text, whether that be an email, a paper document, or an online article.
They're the one's that wrote the text and they should be responsible, accurate in what they say.
If someone hands me a document written by an author that slanders someone, I don't, I can't hold the person who acted as the mailman, to account, he didn't write the document. Surely the resonable thing here is that I sue the guy that wrote the document in the first place.
Is it not the same with web links? If some guy sends me a few links, he's acting as that post man, and it's my choice whether or not I go to those links.
If i find defamatory content on the webpage whose link is in the email, surely, the logical approach is that I sue the guy who wrote the original text on the website?
It matters not what was in the mind of the guy sending me the web links in the email. We're then getting into the area of speculation and what was in his mind when he sent me the email.
You can only sue the guy that wrote the original article. Or that's how it should be in my opinion,