MDL-18903 All outgoing emails can be diverted into a single address

This is highly useful when developing or debugging any emailing feature.
Backported from 2.0dev

config-dist.php

@@ -364,6 +364,8 @@ $CFG->admin = 'admin';
 // When working with production data on test servers, no emails should ever be send to real users
 // $CFG->noemailever = true;
 //
+// Divert all outgoing emails to this address to test and debug emailing features
+// $CFG->divertallemailsto = '[email protected]';
 //
 //=========================================================================
 // ALL DONE!  To continue installation, visit your main page with a browser

lib/moodlelib.php

@@ -4207,6 +4207,12 @@ function email_to_user($user, $from, $subject, $messagetext, $messagehtml='', $a
         return true;
     }
 
+    if (!empty($CFG->divertallemailsto)) {
+        $subject = "[DIVERTED {$user->email}] $subject";
+        $user = clone($user);
+        $user->email = $CFG->divertallemailsto;
+    }
+
     // skip mail to suspended users
     if (isset($user->auth) && $user->auth=='nologin') {
         return true;