nas.core.fs3d.net has no MX records.
Question and response
Question
dig nas.core.fs3d.net. MX
Answer
The response has no answer records.
Authority
core.fs3d.net. 1800 SOA ns1.digitalocean.com. hostmaster.core.fs3d.net. 1660986915 10800 3600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0