I can’t help but think this is a bit user hostile if action is taken based on it, if it’s not one point in a total risk assessment or something.

That’s an incredibly off-base analogy. An e-mail address isn’t a payment form and receiving spam from a company that for no reason wants your e-mail isn’t the same as trading money for a product. I’m sorry if I offended anyone who uses this mildly dark pattern and wants to rationalize it, but you’re definitely moving towards user hostility in doing so, unless you can prove value to your users and show that value as an imperative to using a real e-mail address.

The lookup process seems a bit inefficient, I would pattern match on function heads instead.

for bad_domain <- File.stream!("bad_domains") do
  def lookup(unquote(bad_domain)), do: "EmailGuard.BadDomainList"
end

def lookup(_unmatched), do: nil

def check(email) do
  domain = extract_domain(email)
  if resp = lookup(domain) do
    {:error, resp}
  else
    :ok 
  end 
end