Explanation:

In a scenario where Universal Containers has multiple Salesforce instances, and users receive email links that direct them to specific Salesforce records, it is critical that users are routed to the correct Salesforce org and properly authenticated through their Identity Provider (IdP). To support this behavior, My Domain must be enabled in each Salesforce instance. Enabling My Domain provides a unique, branded login URL (e.g., https://companyA.my.salesforce.com) for each org, which is essential for correctly routing users to the right Salesforce instance, especially in SAML-based SSO or deep-link scenarios.

When a user clicks an email link, the presence of My Domain ensures that Salesforce can identify the correct org and invoke the proper SAML login flow via the configured IdP. Without My Domain, Salesforce uses generic login URLs that do not support deep-linking or SSO redirection properly, leading to potential login issues or redirection failures.

Options like External Identity, Identity Provider, and Multi-Factor Authentication relate to specific security and access use cases, but they are not prerequisites for handling org-specific deep links or SSO routing. My Domain is the foundational requirement that enables this functionality.