mail.siteground.com has no CNAME records.
Question and response
Question
dig mail.siteground.com. NS
Answer
The response has no answer records.
Authority
siteground.com. 360 SOA ns-cloud-a1.googledomains.com. sysadmin.siteground.com. 2012102024 60 60 360 360
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0