smtp.rank.com.co has no NS records.
NS records for this zone are defined at rank.com.co.
NS records for this zone are defined at rank.com.co.
Question and response
Question
dig smtp.rank.com.co. NS
Answer
The response has no answer records.
Authority
rank.com.co. 3600 SOA ns8.wixdns.net. support.wix.com. 2015102813 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0