When open an email with HTML content on "href" images from http content (for example newsletters ) my client browser shows me an alert on the certificate and in some cases it blocks showing the images.
Same email forwarded in another mail system (in cloud) is seen without problems. This happens because in the cloud another mail system is caching the "mixed content" images href behind its HTTPS proxy web servers.
The same behavior with Safari browser on a MAC iOS . It seems in MAC the security is stronger and the href images proposed in HTTP content are more often blocked than Windows OS