mail2.openinvoice.com has no CNAME records.
Question and response
Question
dig mail2.openinvoice.com. NS
Answer
The response has no answer records.
Authority
openinvoice.com. 636 SOA ns-902.awsdns-48.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0