This is impossible to classify as spam using an LLM. The only giveaway is the sender email-address and possibly the links in the document. These should be checked by an agent against a list. It's the only accurate way to solve it.
An LLM can be trained to actively check that every email comes from a domain controlled by the claimed author of the email. That DKIM signing was successful from the purported issuing domain, and so on.
All of the stuff I do when grandma calls asking whether an email is legitimate.
We should have LLMs trained powerfully in detecting signs of these attacks -- after all, we have literally trillions of training examples stored!