smtp.customcall.us has no MX records.
Question and response
Question
dig smtp.customcall.us. MX
Answer
The response has no answer records.
Authority
customcall.us. 86400 SOA pdns99.ultradns.com. rob\.whritenour.enghouse.com. 2015062154 86400 86400 86400 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0