E-mails do not support embedded video playback (e.g. MP4). This is a general limitation across all email clients, not specific to Hello Email.
Alternatives:
Use GIFs: Short animations can be uploaded as GIFs and inserted like an image. They will play automatically in most e-mail clients.
→ Example tool for conversion: ezgif.com/video-to-gif
Link to the video: Host the video on an external platform (website, YouTube, Vimeo, landing page) and link to it from the e-mail. Adding a thumbnail with a play icon often increases clicks.