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Source: AllowedEmailDomain
To enable this object you need to contact Salesforce.com support: I had no way to enable it quickly so I’m just gonna post what this feature means.
The AllowedEmailDomain object is used to store the allowed email domains for your users.
To configure it go to Setup > Manage Users > : you can add domains (e.g. google.com) or even subdomains (e.g. app.enree.co): from now on every new or updated user’s Email field must match the domain whitelist configure in this section.
Existing users’ emails will still be valid till their update.
This restriction doesn’t apply to external users (portal, Communities or Chatter, or simply every user that has a related Contact/Account).