Restrict email address on RHN / Customer Portal accounts to certain domains

Issue

• Add the ability restrict allowable email addresses by domain name
• Business/security requirement that all email from Red Hat be sent to customer's corporate email addresses. Individual users are currently able to change their email address, the Org Admins request the ability to restrict what the email field can be updated to based on a defined filter.
• Provide an account-wide email filter ability, then Org Admins could enable the filter and specify a list of domain or a regex describing what an allowable email address is for their organization.
• If enabled, the Org Admin could specify only email addresses to example.com and example.org are allowed. A user would then try to change their email address to user@hacker.org which would be rejected.