This is an interesting one I’ve just discovered, might be old news, but new to me.
Previously I’d found that animated GIF’s in HTML newsletters didn’t work in most versions of Outlook, instead of just rendering the first frame as they should, I found the pages pretty messed up. So, I’ve avoided using them for ages.
I decided to say feck it and add one today, and in testing with a few versions of Outlook have found that they go render perfectly, and work. Better still, when the recipient mouses off the newsletter the GIF stops and a “play” icon appears in the middle of it. So, you can even turn the GIF into a link, and get a bit more traffic as people try to play it :-)
Do your own testing etc, but from my end, it looks good to go.
This is now getting added to all my CI newsletters: Websites powered by Caffeine ;-)