While this function is counting messages per month, that does not define the actual output of the function, which is writing a summary to the HTTP response.

I think it would be better to have a function that gathers the data by making HTTP calls, and returns the data to a callback function. Then you can write to the response more easily and the function will never need to be passed into it. Like:

	function gatherMailingListSubscriberDetails(startMonth, startYear, callback) {
		// get the info, then call the callback with all the details
		callback(null, {subscriber: 'details'})
	}

	gatherMailingListSubscriberDetails(START_MONTH, START_YEAR, function(err, results) {
		// get the data you need from the results
		// write to the response
	});