diff --git a/addons/mail/static/src/less/thread.less b/addons/mail/static/src/less/thread.less
index 81ab0f917d0..a70d9954a70 100644
--- a/addons/mail/static/src/less/thread.less
+++ b/addons/mail/static/src/less/thread.less
@@ -131,6 +131,7 @@
             }
             img {
                 max-width: 100%;
+                height: auto;
             }
 
             .o_mail_body_long {