rsu.netdesignhost.com has no MX records.
Question and response
Question
dig rsu.netdesignhost.com. MX
Answer
The response has no answer records.
Authority
netdesignhost.com. 3600 SOA win-alfftollgm4. webmaster.netdesignhost.com. 20093198 900 600 86400 3600
Additional
. 0 OPT ; payload 4000, xrcode 0, version 0, flags 0