From WHMCS Documentation
The Email registrar module does not perform any automatic registration/transfer or facilitate any management of domains (update nameservers/whois etc). It could be used when you wish to manually register a domain outside of WHMCS, when using a registrar without a WHMCS module, or if you want to disable domain automation.
When a domain registration, transfer or renewal invoice is paid by a client an email will be sent to staff notifying them manual action is required. The recipient email addresses is specified in the module configuration under Setup > Products/Services > Domain Registrars.