Long explanation in AWS Routing traffic for subdomains – Amazon Route 53

GROUND RULE or RULE OF THUMB of DNS, NEVER change the NS record of your domain in your Domain registrar before completing setup of your DNS server. The result may cause 72 hours of outtage.

Using my domain karmeng.my as an example, the objective is to have chow.karmeng.my to be a valid domain for email hosting which offers customed domain.