The Recipient Control feature restricts users to sending messages to specific e-mail domains. For business-to-business and business-to-consumer scenarios, organizations must restrict their partners and customers to composing messages addressed to their own domain. This prevents partners and customers from using ZixPort to correspond with people in external or unrelated domains.
The default configuration limits sending messages to recipients within your email domain. ZixPort customers can provide a list of domains that portal users can correspond with. The Recipient Control list can be set to a list of all subdomains or just the “parent” domains with all the subdomains included.
This is also needed when Reply to All is configured. It is not case sensitive.