Don't confuse domain names, subdomain names and email accounts. abc@123.com is an email account "abc" at the domain "123.com".

A subdomain would be "abc.123.com", similar to, say, "billing.scam-u.com". If one set up an email account "billing@scam-u.com", this would belong to the primary domain "scam-u.com", not the subdomain "billing.scam-u.com".

Of course, one could set up email accounts for the subdomain: "acodger@billing.scam-u.com".

By "pointing to them", it sounds like they only perform domain name redirection to a hosting space (web page storage) you have set up with another provider; they don't supply the hosting space. That's just a guess--I haven't checked their site.