Once you have setup the Free Email Service for your domain name, you need to modify your Name Servers to the ones specified by www.tnd.vn - Nguyen Ngoc Thanh Trading Limited Company. Generally, the Free Email Service should start working within a span of 24-48 hours post modification of Name Servers.
Do not be confused when you an encounter an error while using some DNS Tools like DNSReport.com:
No NS A records at nameservers
This does not indicate an error and will not affect the Email Service of your domain name.