Set publicly_queryable to true to allow preview of email editor custom post types

See https://developer.wordpress.org/reference/functions/register_post_type/#parameter-detail-information for more info

mailpoet/lib/EmailEditor/Integrations/MailPoet/EmailEditor.php

@@ -67,6 +67,7 @@ class EmailEditor {
           'singular_name' => __('Email', 'mailpoet'),
         ],
         'rewrite' => ['slug' => self::MAILPOET_EMAIL_POST_TYPE],
+        'publicly_queryable' => true,
       ],
     ];
     return $postTypes;