Today, I was told that our users cant send to 1 specific email address with
domain, @sub.domain.com

First of all I try to query the ns record of sub.domain.com but I got no ns
record. However when I do nslookup for mx record, I got the repsond! May I
know is the respond coming from the DNS of "domain.com" but how to have the
mx record working for it??

thks for any ideas,