mx4.limestonenetworks.com has no MX records.
Question and response
Question
dig mx4.limestonenetworks.com. MX
Answer
The response has no answer records.
Authority
limestonenetworks.com. 2560 SOA ns1.limestonenetworks.com. noc.limestonenetworks.com. 1771840769 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0