Add an empty transportsIndex to the mail connector by default

lib/connectors/mail.js

@@ -18,6 +18,7 @@ module.exports = MailConnector;
 function MailConnector(settings) {
   assert(typeof settings === 'object', 'cannot initiaze MailConnector without a settings object');
   var transports = settings.transports || [];
+  this.transportsIndex = {};
 
   transports.forEach(this.setupTransport.bind(this));
 }