mail01.smtpio.com has no MX records.
Question and response
Question
dig mail01.smtpio.com. MX
Answer
The response has no answer records.
Authority
smtpio.com. 1800 SOA isaac.ns.cloudflare.com. dns.cloudflare.com. 2396535168 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0