dc.digicert.com has no MX records.
Question and response
Question
dig dc.digicert.com. MX
Answer
The response has no answer records.
Authority
digicert.com. 180 SOA ns20.digicertdns.com. dns.digicertdns.com. 2009020843 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0