aspmx.sailthru.com has no CNAME records.
Question and response
Question
dig aspmx.sailthru.com. NS
Answer
The response has no answer records.
Authority
sailthru.com. 180 SOA ns10.digicertdns.com. dns.digicertdns.com. 2009010497 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0