To resolve this issue, navigate to Setup -> Session Settings


  1. Under Trusted Domains for Inline Frames click Add Domain.
  2. Copy the current url from your browser address bar
  3. Keep only the domain part which ends with force.com
  4. Ensure to remove the https://
  5. Paste this domain in the Domain field and the iFrame Type should be visualforce
  6. Save


The Email Preview page should now load correctly.