I send an email with an image (jpg) attachment to myself using any of my email accounts, both the sent and received emails show a blue checkmark instead of the image. This only happens on my Mac, and only when I am connected to my Comcast network. If I use my T-Mobil hotspot, a neighbor's wifi, or the xfinitywifi, then the images appear. I've spent many hours with Apple trying to figure out why. We set up a test user, same problem. We created a partition of my Mac's drive and set up a separate "clean" OS, same problem. I just tested sending an email with an image attachment from my webmail, the image attachment shows fine there both sent and received, but in Apple Mail there's the blue checkmarks. Do you have any suggestions how to fix this?