don't send 5 million error messages during daily digest

src/user/digest.js

@@ -9,6 +9,7 @@ var	async = require('async'),
 	user = require('../user'),
 	topics = require('../topics'),
 	batch = require('../batch'),
+	plugins = require('../plugins'),
 	emailer = require('../emailer'),
 	utils = require('../../public/src/utils');
 
@@ -19,6 +20,10 @@ var	async = require('async'),
 			return winston.verbose('[user/jobs] Did not send digests (' + interval + ') because subscription system is disabled.');
 		}
 
+		if (!plugins.hasListeners('action:email.send')) {
+			return winston.error('[user/jobs] Did not send digests (' + interval + ') because no active email plugin was found.');
+		}
+
 		if (!interval) {
 			// interval is one of: day, week, month, or year
 			interval = 'day';