Azure AD B2C allows you to implement custom policies to restrict access based on user attributes, such as an email address.

How It Works:

1. Custom Policy: Modify the B2C user journey to include a validation step that checks if the user's email matches an approved list.
2. Dynamic List: Store the list of approved emails in a centralized place like Azure Table Storage, Azure Cosmos DB, or an external API.
3. Validation Step: During authentication or account creation, Azure B2C calls a REST API endpoint to validate the email.