We are creating a transactional message by email. When someone opens the email, an image that is in the header is not shown but when the same person opens the email a second time, it is displayed, do you know what can the reason be?
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.