Outlook is extra finicky about supporting images and videos, so please use the Static thumbnail option. (Do NOT try to embed a video or use a GIF).
To share a video in Outlook:
1) Record or upload your video.
2) Click Share and then click on Customize
3) At the bottom, click on the middle option Edit Preview
4) And finally, You will see Animated & Static. For this particular case, only use Static
5) Paste into your email draft