Did you know that you can restrict email domains that can be used in a user’s email address in Salesforce.com. For example if you do not want users to change their email address in Salesforce.com to non company email address (i.e. hotmail.com, yahoo.com or gmail.com) you can prevent them from doing so. You will need to contact Salesforce.com to enable this feature and once enabled, users or system administrators will not be able to use any other email than other than the allowed email domains.

Restrict email domain in users email addresses