To add a domain in your account, click on Mail Security.
1. In the Domains menu, click Add domain. The Add Domain page is displayed.
2. Add the following :
Domain name, Domain e-mail administrator, User Name & Password
In the SMTP Deliver Server(s), add your customer server host name of public ip.
Click Save, and now you need to create users.
Please review to following article "How to add users"