_relay.amadeus.com has no MX records.
Question and response
Question
dig _relay.amadeus.com. MX
Answer
The response has no answer records.
Authority
amadeus.com. 600 SOA mucdns01.amadeus-dns.com. nicadmin.amadeus.com. 2008039968 3600 1200 2592000 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0