What It Is — and What Should Be In It
For debts claimed by a business against an individual, the Pre-Action Protocol for Debt Claims requires the letter to include: how much is claimed and how it's calculated, who the original creditor was if the debt was sold, an Information Sheet and Reply Form, and a Financial Statement form. You then normally have 30 days to reply before they can reasonably start a claim.
A threatening email with none of that attached isn't protocol-compliant — which matters, because courts expect the protocol to be followed.