diff --git a/addons/mail/static/src/less/thread.less b/addons/mail/static/src/less/thread.less
index 05db1bdbfa9..76fccd4e012 100644
--- a/addons/mail/static/src/less/thread.less
+++ b/addons/mail/static/src/less/thread.less
@@ -135,6 +135,7 @@
             }
             img {
                 max-width: 100%;
+                height: auto;
             }
 
             .o_mail_body_long {