mail.rivnet.com has no MX records.
Question and response
Question
dig mail.rivnet.com. MX
Answer
The response has no answer records.
Authority
rivnet.com. 900 SOA nsb.vtx1.net. fieldopadmin.vtx1.net. 160 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0