_ext.constantcontact.com has no MX records.
Question and response
Question
dig _ext.constantcontact.com. MX
Answer
The response has no answer records.
Authority
constantcontact.com. 600 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1657679372 3600 600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0