mbssmtp.mbsbooks.com has no MX records.
Question and response
Question
dig mbssmtp.mbsbooks.com. MX
Answer
The response has no answer records.
Authority
mbsbooks.com. 1800 SOA chan.ns.cloudflare.com. dns.cloudflare.com. 2397622840 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0