diff --git a/classes/form/preferences_form.php b/classes/form/preferences_form.php
index 40c59bbb..def06ec4 100644
--- a/classes/form/preferences_form.php
+++ b/classes/form/preferences_form.php
@@ -23,7 +23,7 @@
 
 namespace local_extension\form;
 
-use local_extension\preferences_manager;
+use local_extension\preferences;
 use moodleform;
 
 defined('MOODLE_INTERNAL') || die();
@@ -33,10 +33,10 @@
 
 class preferences_form extends moodleform {
     public function definition() {
-        $this->_form->addElement('checkbox', preferences_manager::MAIL_DIGEST,
+        $this->_form->addElement('checkbox', preferences::MAIL_DIGEST,
                                  get_string('preference_mail_digest', 'local_extension'),
                                  get_string('preference_mail_digest_help', 'local_extension'));
-        $this->_form->setDefault('mail_digest', preferences_manager::get(preferences_manager::MAIL_DIGEST));
+        $this->_form->setDefault('mail_digest', (new preferences())->get(preferences::MAIL_DIGEST));
 
         $this->add_action_buttons();
     }
diff --git a/classes/message/mailer.php b/classes/message/mailer.php
index 2eafa372..c2fb3b30 100644
--- a/classes/message/mailer.php
+++ b/classes/message/mailer.php
@@ -23,10 +23,15 @@
 
 namespace local_extension\message;
 
+use local_extension\preferences;
+
 defined('MOODLE_INTERNAL') || die();
 
 class mailer {
     public function send($message) {
-        message_send($message);
+        $preferences = new preferences();
+        if (!$preferences->get(preferences::MAIL_DIGEST)) {
+            message_send($message);
+        }
     }
 }
diff --git a/classes/preferences_manager.php b/classes/preferences.php
similarity index 78%
rename from classes/preferences_manager.php
rename to classes/preferences.php
index fc022c80..d71612b5 100644
--- a/classes/preferences_manager.php
+++ b/classes/preferences.php
@@ -31,19 +31,36 @@
  * @copyright   2018 Catalyst IT Australia {@link http://www.catalyst-au.net}
  * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-abstract class preferences_manager {
+class preferences {
     const MAIL_DIGEST = 'mail_digest';
 
     protected static $defaults = [
         self::MAIL_DIGEST => false,
     ];
 
-    public static function set($name, $value) {
-        set_user_preferences(["local_extension_{$name}" => $value]);
+    /** @var int */
+    private $userid;
+
+    public function get_user_id() {
+        return $this->userid;
     }
 
-    public static function get($name) {
+    public function __construct($userid = null) {
+        global $USER;
+
+        if (is_null($userid)) {
+            $userid = $USER->id;
+        }
+
+        $this->userid = $userid;
+    }
+
+    public function get($name) {
         $default = array_key_exists($name, self::$defaults) ? self::$defaults[$name] : null;
-        return get_user_preferences("local_extension_{$name}", $default);
+        return get_user_preferences("local_extension_{$name}", $default, $this->userid);
+    }
+
+    public function set($name, $value) {
+        set_user_preferences(["local_extension_{$name}" => $value], $this->userid);
     }
 }
diff --git a/preferences.php b/preferences.php
index 8606a1c2..c9f0d4ac 100644
--- a/preferences.php
+++ b/preferences.php
@@ -24,7 +24,7 @@
  */
 
 use local_extension\form\preferences_form;
-use local_extension\preferences_manager;
+use local_extension\preferences;
 
 require_once(__DIR__ . '/../../config.php');
 
@@ -48,7 +48,7 @@
 $data = $form->get_data();
 
 if (!is_null($data)) {
-    preferences_manager::set(preferences_manager::MAIL_DIGEST, !empty($data->{preferences_manager::MAIL_DIGEST}));
+    (new preferences())->set(preferences::MAIL_DIGEST, !empty($data->{preferences::MAIL_DIGEST}));
     echo $OUTPUT->notification(get_string('preferences_saved', 'local_extension'), 'notifysuccess');
 }
 
diff --git a/tests/phpunit/message/mailer_test.php b/tests/phpunit/message/mailer_test.php
index d816e55f..b84014fd 100644
--- a/tests/phpunit/message/mailer_test.php
+++ b/tests/phpunit/message/mailer_test.php
@@ -22,6 +22,8 @@
  */
 
 use core\message\message;
+use local_extension\message\mailer;
+use local_extension\preferences;
 use local_extension\test\extension_testcase;
 
 defined('MOODLE_INTERNAL') || die();
@@ -33,9 +35,17 @@
  * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 class local_extension_mailer_test extends extension_testcase {
-    protected function send_message() {
+    private $recipient;
+
+    protected function setUp() {
+        parent::setUp();
+        self::resetAfterTest();
+        $this->recipient = $this->getDataGenerator()->create_user(['email' => 'destination@extension.test']);
+    }
+
+    protected function send_email() {
         global $CFG;
-        $user = core_user::get_user(2);
+        $user = $this->recipient;
 
         if ($CFG->version >= 2015051100) {
             $message = new message();
@@ -52,14 +62,14 @@ protected function send_message() {
         $message->fullmessage = 'Message Contents';
         $message->fullmessageformat = FORMAT_PLAIN;
 
-        message_send($message);
+        (new mailer())->send($message);
     }
 
     public function test_it_sends_message_immediately() {
-        $this->resetAfterTest();
+        self::setAdminUser();
 
         $sink = phpunit_util::start_message_redirection();
-        $this->send_message();
+        $this->send_email();
         phpunit_util::stop_message_redirection();
 
         $messages = $sink->get_messages();
@@ -68,4 +78,18 @@ public function test_it_sends_message_immediately() {
         self::assertSame('Message Subject', $message->subject);
         self::assertSame('Message Contents', $message->fullmessage);
     }
+
+    public function test_it_does_not_send_message_immediately() {
+        $this->resetAfterTest();
+        self::setAdminUser();
+
+        (new preferences())->set(preferences::MAIL_DIGEST, true);
+
+        $sink = phpunit_util::start_message_redirection();
+        $this->send_email();
+        phpunit_util::stop_message_redirection();
+
+        $messages = $sink->get_messages();
+        self::assertCount(0, $messages);
+    }
 }
diff --git a/tests/phpunit/preferences_manager_test.php b/tests/phpunit/preferences_test.php
similarity index 53%
rename from tests/phpunit/preferences_manager_test.php
rename to tests/phpunit/preferences_test.php
index 15f06818..9dc10803 100644
--- a/tests/phpunit/preferences_manager_test.php
+++ b/tests/phpunit/preferences_test.php
@@ -21,7 +21,7 @@
  * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
 
-use local_extension\preferences_manager;
+use local_extension\preferences;
 use local_extension\test\extension_testcase;
 
 defined('MOODLE_INTERNAL') || die();
@@ -32,25 +32,58 @@
  * @copyright   2017 Catalyst IT Australia {@link http://www.catalyst-au.net}
  * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
  */
-class local_extension_preferences_manager_test extends extension_testcase {
-    public function test_it_sets() {
-        preferences_manager::set('name', 'abc123');
+class local_extension_preferences_test extends extension_testcase {
+    protected function setUp() {
+        parent::setUp();
+        $this->resetAfterTest();
+        self::setAdminUser();
+    }
+
+    public function test_it_creates_with_userid() {
+        global $USER;
+
+        $tests = [
+            [null, $USER->id],
+            [999, 999],
+        ];
+
+        foreach ($tests as $key => list($parameter, $expected)) {
+            $manager = new preferences($parameter);
+            self::assertSame($expected, $manager->get_user_id(), "Test #{$key}");
+        }
+    }
+
+    public function test_it_sets_for_current_user() {
+        (new preferences())->set('name', 'abc123');
         self::assertSame('abc123', get_user_preferences('local_extension_name'));
     }
 
-    public function test_it_gets() {
+    public function test_it_gets_for_current_user() {
         set_user_preferences(['local_extension_name' => 'john']);
-        $got = preferences_manager::get('name');
+        $got = (new preferences())->get('name');
+        self::assertSame('john', $got);
+    }
+
+    public function test_it_sets_for_another_user() {
+        $user = $this->getDataGenerator()->create_user();
+        (new preferences($user->id))->set('name', 'abc123');
+        self::assertSame('abc123', get_user_preferences('local_extension_name', null, $user));
+    }
+
+    public function test_it_gets_for_another_user() {
+        $user = $this->getDataGenerator()->create_user();
+        set_user_preferences(['local_extension_name' => 'john'], $user);
+        $got = (new preferences($user->id))->get('name');
         self::assertSame('john', $got);
     }
 
     public function test_it_has_defaults() {
         $expectations = [
-            'invalid'                        => null,
-            preferences_manager::MAIL_DIGEST => false,
+            'invalid'                => null,
+            preferences::MAIL_DIGEST => false,
         ];
         foreach ($expectations as $preference => $expectation) {
-            $actual = preferences_manager::get($preference);
+            $actual = (new preferences())->get($preference);
             self::assertSame($expectation, $actual, "Preference: {$preference}");
         }
     }