ns4b.dhl.com has no MX records.
Question and response
Question
dig ns4b.dhl.com. MX
Answer
The response has no answer records.
Authority
dhl.com. 300 SOA ns4.dhl.com. netmaster.dhl.com. 2016118527 10800 3600 604800 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0