www.optimum.net has no MX records.
Question and response
Question
dig www.optimum.net. MX
Answer
The response has no answer records.
Authority
optimum.net. 7200 SOA authns1.cv.net. postmaster.optimum.net. 2013141209 7200 600 604800 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0