rev.booksys.com has no MX records.
Question and response
Question
dig rev.booksys.com. MX
Answer
The response has no answer records.
Authority
rev.booksys.com. 3600 SOA ns1.booksys.com. dns.booksys.com. 22 86400 600 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0