async fn newsletter_subscribe<P: NewsletterSubscriptionProvider, M: Mailer + 'static>(
    // Cannot currently use Typed headers, as Referer does not (yet) implement Display: https://github.com/hyperium/headers/pull/122
    // TypedHeader(user_agent): TypedHeader<UserAgent>,
    // TypedHeader(referrer): TypedHeader<axum::headers::Referer>,
    headers: HeaderMap,
    InsecureClientIp(user_ip): InsecureClientIp,
    State(state): State<Arc<NewsletterState<P, M>>>,
    ValidatedJson(payload): ValidatedJson<SubscribeRequest>,
) -> Result<impl IntoResponse> {
    let referrer = match headers.get(header::REFERER) {
        Some(header_value) => header_value
            .to_str()
            .map_err(ClientError::HeaderToStrError)?,
        None => "https://www.example.com",
    };
    let user_agent = match headers.get(header::USER_AGENT) {
        Some(header_value) => header_value
            .to_str()
            .map_err(ClientError::HeaderToStrError)?,
        None => "",
    };