mail.mailersend.com has no MX records.
Question and response
Question
dig mail.mailersend.com. MX
Answer
The response has no answer records.
Authority
mailersend.com. 1800 SOA andy.ns.cloudflare.com. dns.cloudflare.com. 2397003248 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0