diff --git a/src/applications/config/check/PhabricatorMailSetupCheck.php b/src/applications/config/check/PhabricatorMailSetupCheck.php
--- a/src/applications/config/check/PhabricatorMailSetupCheck.php
+++ b/src/applications/config/check/PhabricatorMailSetupCheck.php
@@ -7,6 +7,10 @@
   }
 
   protected function executeChecks() {
+    if (PhabricatorEnv::getEnvConfig('cluster.mailers')) {
+      return;
+    }
+
     $adapter = PhabricatorEnv::getEnvConfig('metamta.mail-adapter');
 
     switch ($adapter) {
diff --git a/src/infrastructure/cluster/config/PhabricatorClusterMailersConfigType.php b/src/infrastructure/cluster/config/PhabricatorClusterMailersConfigType.php
--- a/src/infrastructure/cluster/config/PhabricatorClusterMailersConfigType.php
+++ b/src/infrastructure/cluster/config/PhabricatorClusterMailersConfigType.php
@@ -63,8 +63,8 @@
       }
       $map[$key] = true;
 
-      $priority = idx($spec, 'priority', 0);
-      if ($priority <= 0) {
+      $priority = idx($spec, 'priority');
+      if ($priority !== null && $priority <= 0) {
         throw $this->newException(
           pht(
             'Mailer configuration ("%s") is invalid: priority must be '.