mailfilter1.rackhosting.com has no CNAME records.
Question and response
Question
dig mailfilter1.rackhosting.com. NS
Answer
The response has no answer records.
Authority
rackhosting.com. 393 SOA dns1.rackhosting.com. drift.rackhosting.com. 1773063859 10800 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0