Issue #3214505 by lawxen: $to can't be empty string

src/Plugin/Mail/SMTPMailSystem.php

@@ -291,6 +291,9 @@ class SMTPMailSystem implements MailInterface, ContainerFactoryPluginInterface {
     // Create the list of 'To:' recipients.
     $torecipients = explode(',', $to);
     foreach ($torecipients as $torecipient) {
+      if (empty($torecipient)) {
+        continue;
+      }
       $to_comp = $this->getComponents($torecipient);
       $mailer->AddAddress($to_comp['email'], $to_comp['name']);
     }