mail.oo155.com has no NS records.
NS records for this zone are defined at oo155.com.
NS records for this zone are defined at oo155.com.
Question and response
Question
dig mail.oo155.com. NS
Answer
The response has no answer records.
Authority
oo155.com. 900 SOA ns-389.awsdns-48.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0