diff --git a/addons/mail/static/src/scss/thread.scss b/addons/mail/static/src/scss/thread.scss
index 05f5517f404..ef220afa06c 100644
--- a/addons/mail/static/src/scss/thread.scss
+++ b/addons/mail/static/src/scss/thread.scss
@@ -183,7 +183,7 @@
                 display: inline-block;
                 word-break: break-all;
             }
-            img {
+            :not(.o_image_box) > img {
                 max-width: 100%;
                 height: auto;
             }