diff --git a/addons/website_mass_mailing/static/src/js/website_mass_mailing.js b/addons/website_mass_mailing/static/src/js/website_mass_mailing.js
index c13aa9dcda2..f09476e1a5a 100644
--- a/addons/website_mass_mailing/static/src/js/website_mass_mailing.js
+++ b/addons/website_mass_mailing/static/src/js/website_mass_mailing.js
@@ -140,4 +140,25 @@ publicWidget.registry.subscribe = publicWidget.Widget.extend({
         });
     },
 });
+
+/**
+ * This widget tries to fix snippets that were malformed because of a missing
+ * upgrade script. Without this, some newsletter snippets coming from users
+ * upgraded from a version lower than 16.0 may not be able to update their
+ * newsletter block.
+ *
+ * TODO an upgrade script should be made to fix databases and get rid of this.
+ */
+publicWidget.registry.fixNewsletterListClass = publicWidget.Widget.extend({
+    selector: '.s_newsletter_subscribe_form:not(.s_subscription_list), .s_newsletter_block',
+
+    /**
+     * @override
+     */
+    start() {
+        this.$target[0].classList.add('s_newsletter_list');
+        return this._super(...arguments);
+    },
+});
+
 });
diff --git a/addons/website_mass_mailing/views/snippets_templates.xml b/addons/website_mass_mailing/views/snippets_templates.xml
index 14552219114..4949f08708f 100644
--- a/addons/website_mass_mailing/views/snippets_templates.xml
+++ b/addons/website_mass_mailing/views/snippets_templates.xml
@@ -23,6 +23,10 @@
     </xpath>
 </template>
 
+<!--
+Users upgraded from a version lower than 16.0 may have those blocks in their
+database, without the s_newsletter_list class. See fixNewsletterListClass.
+-->
 <template id="s_newsletter_subscribe_form" name="Newsletter">
     <div class="s_newsletter_subscribe_form s_newsletter_list js_subscribe" data-vxml="001" data-list-id="0" data-name="Newsletter Form">
         <div class="input-group">
@@ -33,6 +37,10 @@
     </div>
 </template>
 
+<!--
+Users upgraded from a version lower than 16.0 may have those blocks in their
+database, without the s_newsletter_list class. See fixNewsletterListClass.
+-->
 <template id="s_newsletter_block" name="Newsletter block">
     <section class="s_newsletter_block s_newsletter_list pt32 pb32" data-list-id="0">
         <div class="container">