nb.network-box.com has no MX records.
Question and response
Question
dig nb.network-box.com. MX
Answer
The response has no answer records.
Authority
network-box.com. 600 SOA nb.network-box.com. postmaster.network-box.com. 2025102800 3600 7200 604800 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0