mx.rs.eserver.net has no CNAME records.
Question and response
Question
dig mx.rs.eserver.net. NS
Answer
The response has no answer records.
Authority
rs.eserver.net. 1752 SOA ns1.rs.eserver.net. hostmaster.rs.eserver.net. 2025082401 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0