diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -3178,6 +3178,7 @@
     'PhabricatorMacroTransactionQuery' => 'applications/macro/query/PhabricatorMacroTransactionQuery.php',
     'PhabricatorMacroTransactionType' => 'applications/macro/xaction/PhabricatorMacroTransactionType.php',
     'PhabricatorMacroViewController' => 'applications/macro/controller/PhabricatorMacroViewController.php',
+    'PhabricatorMailConfigTestCase' => 'applications/metamta/storage/__tests__/PhabricatorMailConfigTestCase.php',
     'PhabricatorMailEmailHeraldField' => 'applications/metamta/herald/PhabricatorMailEmailHeraldField.php',
     'PhabricatorMailEmailHeraldFieldGroup' => 'applications/metamta/herald/PhabricatorMailEmailHeraldFieldGroup.php',
     'PhabricatorMailEmailSubjectHeraldField' => 'applications/metamta/herald/PhabricatorMailEmailSubjectHeraldField.php',
@@ -8680,6 +8681,7 @@
     'PhabricatorMacroTransactionQuery' => 'PhabricatorApplicationTransactionQuery',
     'PhabricatorMacroTransactionType' => 'PhabricatorModularTransactionType',
     'PhabricatorMacroViewController' => 'PhabricatorMacroController',
+    'PhabricatorMailConfigTestCase' => 'PhabricatorTestCase',
     'PhabricatorMailEmailHeraldField' => 'HeraldField',
     'PhabricatorMailEmailHeraldFieldGroup' => 'HeraldFieldGroup',
     'PhabricatorMailEmailSubjectHeraldField' => 'PhabricatorMailEmailHeraldField',
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
@@ -513,12 +513,14 @@
         $defaults = $mailer->newDefaultOptions();
         $options = idx($spec, 'options', array()) + $defaults;
         $mailer->setOptions($options);
+
+        $mailers[] = $mailer;
       }
     }
 
     $sorted = array();
     $groups = mgroup($mailers, 'getPriority');
-    ksort($groups);
+    krsort($groups);
     foreach ($groups as $group) {
       // Reorder services within the same priority group randomly.
       shuffle($group);
diff --git a/src/applications/metamta/storage/__tests__/PhabricatorMailConfigTestCase.php b/src/applications/metamta/storage/__tests__/PhabricatorMailConfigTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/applications/metamta/storage/__tests__/PhabricatorMailConfigTestCase.php
@@ -0,0 +1,131 @@
+<?php
+
+final class PhabricatorMailConfigTestCase
+  extends PhabricatorTestCase {
+
+  public function testMailerPriorities() {
+    $mailers = $this->newMailersWithConfig(
+      array(
+        array(
+          'key' => 'A',
+          'type' => 'test',
+        ),
+        array(
+          'key' => 'B',
+          'type' => 'test',
+        ),
+      ));
+
+    $this->assertEqual(
+      array('A', 'B'),
+      mpull($mailers, 'getKey'));
+
+    $mailers = $this->newMailersWithConfig(
+      array(
+        array(
+          'key' => 'A',
+          'priority' => 1,
+          'type' => 'test',
+        ),
+        array(
+          'key' => 'B',
+          'priority' => 2,
+          'type' => 'test',
+        ),
+      ));
+
+    $this->assertEqual(
+      array('B', 'A'),
+      mpull($mailers, 'getKey'));
+
+    $mailers = $this->newMailersWithConfig(
+      array(
+        array(
+          'key' => 'A1',
+          'priority' => 300,
+          'type' => 'test',
+        ),
+        array(
+          'key' => 'A2',
+          'priority' => 300,
+          'type' => 'test',
+        ),
+        array(
+          'key' => 'B',
+          'type' => 'test',
+        ),
+        array(
+          'key' => 'C',
+          'priority' => 400,
+          'type' => 'test',
+        ),
+        array(
+          'key' => 'D',
+          'type' => 'test',
+        ),
+      ));
+
+    // The "A" servers should be shuffled randomly, so either outcome is
+    // acceptable.
+    $option_1 = array('C', 'A1', 'A2', 'B', 'D');
+    $option_2 = array('C', 'A2', 'A1', 'B', 'D');
+    $actual = mpull($mailers, 'getKey');
+
+    $this->assertTrue(($actual === $option_1) || ($actual === $option_2));
+
+    // Make sure that when we're load balancing we actually send traffic to
+    // both servers reasonably often.
+    $saw_a1 = false;
+    $saw_a2 = false;
+    $attempts = 0;
+    while (true) {
+      $mailers = $this->newMailersWithConfig(
+        array(
+          array(
+            'key' => 'A1',
+            'priority' => 300,
+            'type' => 'test',
+          ),
+          array(
+            'key' => 'A2',
+            'priority' => 300,
+            'type' => 'test',
+          ),
+        ));
+
+      $first_key = head($mailers)->getKey();
+
+      if ($first_key == 'A1') {
+        $saw_a1 = true;
+      }
+
+      if ($first_key == 'A2') {
+        $saw_a2 = true;
+      }
+
+      if ($saw_a1 && $saw_a2) {
+        break;
+      }
+
+      if ($attempts++ > 1024) {
+        throw new Exception(
+          pht(
+            'Load balancing between two mail servers did not select both '.
+            'servers after an absurd number of attempts.'));
+      }
+    }
+
+    $this->assertTrue($saw_a1 && $saw_a2);
+  }
+
+  private function newMailersWithConfig(array $config) {
+    $env = PhabricatorEnv::beginScopedEnv();
+    $env->overrideEnvConfig('cluster.mailers', $config);
+
+    $mailers = PhabricatorMetaMTAMail::newMailers();
+
+    unset($env);
+    return $mailers;
+  }
+
+}