When creating promo codes in Brandibble, you have the ability to restrict their use to individuals with email addresses ending in certain domains. For example, if you're offering Google employees a discount, you'd add google.com to the Domain field. Only customers with email addresses ending in @google.com will be able to use that promo code.
In order to add this restriction, set up the promo code and fill out all relevant fields. In the Domain field, add the domain you'd like to use and click Add Promo Code.
This feature is not to be confused with the Approved Email Address restriction, which will only allow customers with specified email addresses use the promo codes. If you select Yes for Approved Email Address, you'll have to specify what those email addresses are in the Approved Emails tab.