mail.xvserver.com has no NS records.
NS records for this zone are defined at xvserver.com.
NS records for this zone are defined at xvserver.com.
Question and response
Question
dig mail.xvserver.com. NS
Answer
The response has no answer records.
Authority
xvserver.com. 14400 SOA ns1.xvserver.com. root.xvserver.com. 2017060700 14400 3600 1209600 86400
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0