reply-mx.s6.exacttarget.com has no SRV records.
Question and response
Question
dig reply-mx.s6.exacttarget.com. SRV
Answer
The response has no answer records.
Authority
s6.exacttarget.com. 60 SOA dns1.p01.nsone.net. hostmaster.nsone.net. 1680013781 7200 3600 1209600 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0