code: function verifyMailgun(apikey, token, timestamp, signature) {
    var data = [timestamp, token].join('');
    var hmac = crypto.createHmac('sha256', apikey).update(data);
    var ourSig = hmac.digest('hex');
  
    var output = {
      apikey: apikey,
      token: token,
      timestamp: timestamp,
      signature: signature,
      generatedSignature: ourSig,
      valid: ourSig === signature
    };
  
    return output;
  }
docs: Verify mailgun HMAC authentication token