dkim.smtp2go.net has no MX records.
Question and response
Question
dig dkim.smtp2go.net. MX
Answer
The response has no answer records.
Authority
smtp2go.net. 1800 SOA ada.ns.cloudflare.com. dns.cloudflare.com. 2393946122 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0