There's another community post with 404 debugging tips that might be helpful. Please give these solutions a try and let us know how it goes.

https://community.vercel.com/t/debugging-404-errors/437

Vercel Community

Debugging 404 Errors

Sometimes things don’t go as expected when deploying a website. If you see a 404 on your site, that means the page or resource couldn’t be found. There are...

A human should be around soon to offer more advice. But you can also get helpful information quickly by asking v0.

Hi @mickybizzle, welcome to the Vercel Community!

The link you shared here, seems to be a preview deployment that's protected by the https://vercel.com/docs/security/deployment-protection. Hence, only you or your team members can access this deployment (and the image).

I'd recommend promoting the deployment to the production environment and using that link in the email.

I hope this was helpful.