webmail.optimum.net has no CNAME records.
Question and response
Question
dig webmail.optimum.net. NS
Answer
The response has no answer records.
Authority
optimum.net. 6796 SOA authns1.cv.net. postmaster.optimum.net. 2013141209 7200 600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0