sf._domainkey.ubermetrics.com has no NS records.
NS records for this zone are defined at ubermetrics.com.
NS records for this zone are defined at ubermetrics.com.
Question and response
Question
dig sf.f5o721.custdkim.salesforce.com. NS
Answer
The response has no answer records.
Authority
custdkim.salesforce.com. 30 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2041201157 1800 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0