To restrict the email domains allowed in a user’s Email field, enable the email domain allowlist. Previously, you contacted Salesforce Support to enable this feature.
Where: This change applies to Lightning Experience and Salesforce Classic in all editions.
How: From Setup, in the Quick Find Box, enter User Management, and then select User Management Settings. Turn on Email Domain Allowlist. Then manage the permitted email domains for users from the Allowed Email Domains Setup page.