spf.constantcontact.com has no MX records.
Question and response
Question
dig spf.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