diff --git a/src/applications/metamta/adapter/PhabricatorMailImplementationPHPMailerAdapter.php b/src/applications/metamta/adapter/PhabricatorMailImplementationPHPMailerAdapter.php
index 6581e56753..3af3fbb1e0 100644
--- a/src/applications/metamta/adapter/PhabricatorMailImplementationPHPMailerAdapter.php
+++ b/src/applications/metamta/adapter/PhabricatorMailImplementationPHPMailerAdapter.php
@@ -33,6 +33,7 @@ final class PhabricatorMailImplementationPHPMailerAdapter
 
       $protocol = PhabricatorEnv::getEnvConfig('phpmailer.smtp-protocol');
       if ($protocol) {
+        $protocol = phutil_utf8_strtolower($protocol);
         $this->mailer->SMTPSecure = $protocol;
       }
     } else if ($mailer == 'sendmail') {