diff --git a/addons/mass_mailing/static/src/scss/mass_mailing.ui.scss b/addons/mass_mailing/static/src/scss/mass_mailing.ui.scss
index 18deed63f25..429880307d3 100644
--- a/addons/mass_mailing/static/src/scss/mass_mailing.ui.scss
+++ b/addons/mass_mailing/static/src/scss/mass_mailing.ui.scss
@@ -317,6 +317,9 @@ body.editor_enable.o_basic_theme.o_in_iframe {
         color: #999999;
         margin: 5px;
         height: auto;
+        width: -moz-available;          /* WebKit-based browsers will ignore this. */
+        width: -webkit-fill-available;  /* Mozilla-based browsers will ignore this. */
+        width: stretch;
 
         &:before {
             content: attr(data-editor-message);