mx.smtp.dev has no MX records.
Question and response
Question
dig mx.smtp.dev. MX
Answer
The response has no answer records.
Authority
smtp.dev. 1800 SOA curt.ns.cloudflare.com. dns.cloudflare.com. 2397007260 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0