mx20.inbound.createsend.com has no SRV records.
Question and response
Question
dig mx20.inbound.createsend.com. SRV
Answer
The response has no answer records.
Authority
createsend.com. 180 SOA ns10.digicertdns.com. dns.digicertdns.com. 2009010160 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0