mail5.slfs.com has no CNAME records.
Question and response
Question
dig mail5.slfs.com. NS
Answer
The response has no answer records.
Authority
slfs.com. 10800 SOA ns1.markmonitor.com. hostmaster.markmonitor.com. 2026022601 86400 3600 2592000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0