diff -ru src/Symfony/Component/Mailer/Transport/Smtp/SmtpTransport.php src/Symfony/Component/Mailer/Transport/Smtp/SmtpTransport.php
--- src/Symfony/Component/Mailer/Transport/Smtp/SmtpTransport.php	2022-05-12 17:47:20.384666817 +0000
+++ src/Symfony/Component/Mailer/Transport/Smtp/SmtpTransport.php	2022-05-12 17:47:21.978700156 +0000
@@ -161,10 +161,10 @@
     {
         $regexps = [
             '/250 Ok (?P<id>[0-9a-f-]+)\r?$/mis',
-            '/250 Ok:? queued as (?P<id>[A-Z0-9]+)\r?$/mis'
+            '/250 Ok:? queued as (?P<id>[A-Z0-9]+)\r?$/mis',
         ];
 
-        if ($mtaResult === '') {
+        if ('' === $mtaResult) {
             return null;
         }