Why should you not delete onice.io?
The Onice.io domain is a domain that must not be deleted for several reasons.
The main reason is that it is associated with a virtual server and is a unique identifier; it also has DNS records set to be used as a production or test domain. It does not have to have a single user, a single group, or a single resource on it. Whenever you send mail, the recipient's server tries to verify your identity. The server will take your IP, and by using PTR, it will verify your server; without this verification recipient’s server will refuse your email.
Do I have to use the onice.io domain?
No, you don't have to use it. It is a system domain that can stay completely empty and unused. Instead, you can use your own domain that you purchased.
How do I create a new domain?
If you want to use your own domain, you can set it up in WebAdmin -> "+" sign on the top left -> New Domain. You can use this domain however you want and create users, limits, and rules. Creating users depends on your number of purchased licenses.