Yes, obviously, not every such person will have access to company headed notepaper, which is why I mentioned that it's good if there's some way to prove that they are indeed an employee. Probably a letter from their own address is more credible than an email. It's for evidence. Anybody could write an email. As to how INZ will contact them, we have heard of its being by letter, by phone, and even by personal visit (but that last was when there was, rightly, some suspicion over who the person was).