diff --git a/addons/mass_mailing/static/src/js/mass_mailing_snippets.js b/addons/mass_mailing/static/src/js/mass_mailing_snippets.js
index 1ce86fda384..22a216a1830 100644
--- a/addons/mass_mailing/static/src/js/mass_mailing_snippets.js
+++ b/addons/mass_mailing/static/src/js/mass_mailing_snippets.js
@@ -220,6 +220,9 @@ options.registry.DesignTab = options.Class.extend({
             cssTexts.push(rule.cssText);
         }
         this.styleElement.textContent = cssTexts.join('\n');
+        // Flush the rules cache for convert_inline, to make sure they are
+        // recomputed to account for the change.
+        this.options.wysiwyg._rulesCache = undefined;
     },
     /**
      * @override