mail.web.ptd.net has no CNAME records.
Question and response
Question
dig mail.web.ptd.net. NS
Answer
The response has no answer records.
Authority
web.ptd.net. 9575 SOA dns1.ptd.net. dns-request.ptd.net. 2024112601 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0