When sending out emails with videos (attached as hyperlinked thumbnail images), it is important to consider the security settings and filters of yours recipients.
Firewalls/filters that are set to block out hyperlinked images (video thumbnails) or just images in general (including icon & images in signatures) may cause this problem.
Some factors that may cause this issue are as follows:
- the organization's network firewalls,
- the email platforms security settings, as well as the version of their email client.
That being said, even without the video thumbnail, the email should still send through the URL link after the image which users can click or copy and paste to see the video.