Did you know that you can restrict email domains that can be used in a user’s email address in Salesforce?
For example, if you do not want admins to add a user with a non-company email address like gmail.com, hotmail.com or yahoo.com you can prevent them from doing so.
For this, first you need to set the “Allowed Email Domains” in Setup.

And now when the admin tries to add a user with an email domain that is not specified under allowed email domains, they are going to get the error.

This rule is not enforced for existing users. But if edit an existing user, you will need to update the email field to match an allowed email domain.
References & Useful URLs
- Salesforce help article – Restrict User Email Domains
Thanks for Information, it saved my time