a4i0.smtp2go.com has no MX records.
Question and response
Question
dig a4i0.smtp2go.com. MX
Answer
The response has no answer records.
Authority
smtp2go.com. 600 SOA ns2.smtpcorp.com. hostmaster.smtp2go.com. 2015074660 14400 3600 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0