Does it happen only in the mail client or also on the mirror page?If on
the mirror page, you could check opening the network console and check
on the HTTP status of the image display. ACS uses a load balancer to
deliver images.There indeed could be some issue on the instance, that
the image for some reason was only deployed on 1 of the instances
between the load balancer.For that, you may see a 404 in the HTTP
response. In that case, indeed a support ticket is needed to check/fix
the backend.