smartmail.neustar.com has no MX records.
Question and response
Question
dig smartmail.neustar.com. MX
Answer
The response has no answer records.
Authority
neustar.com. 86400 SOA udns1.ultradns.net. networkers.neustar.biz. 1311363932 86400 86400 86400 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0