Domains

Redirections end in sandboxes for instanced URLs in API traffic. And if a third party serves your custom domain, update the target host name.

• Replace Instanced URLs in API Traffic
  To prevent disruption after infrastructure updates that change your instance name, replace instanced URLs in API traffic with your org’s My Domain login URL. That traffic stops in sandboxes in this release.
• Manage Custom Domains More Easily by Using a Stable Target Host Name
  If you use a third-party proxy or CDN service and your org uses Salesforce Edge Network, work with your third-party provider to update the domain's target host name to your org's My Domain login URL.