lon-u-smtp.diligent.com has no MX records.
Question and response
Question
dig lon-u-smtp.diligent.com. MX
Answer
The response has no answer records.
Authority
diligent.com. 1800 SOA ns1-07.azure-dns.com. itservices.boardbooks.com. 2020092400 3600 600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0