Mailbox App for Mac is Not Playing GIFs
My tests are limited, but I tried a GIF that I created and one from reactiongifs.com and I get the same results. Mailbox for Mac is displaying the image at various points as you visit/revisit the email, but never "plays" the GIF.
Here's a GIF of a GIF not working. #meta
Mailbox on iOS 8 works as expected.