stats.uptimerobot.com has no MX records.
Question and response
Question
dig stats.uptimerobot.com. MX
Answer
The response has no answer records.
Authority
uptimerobot.com. 1800 SOA cruz.ns.cloudflare.com. dns.cloudflare.com. 2397738281 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0