smtp01.nfon.net has no MX records.
Question and response
Question
dig smtp01.nfon.net. MX
Answer
The response has no answer records.
Authority
nfon.net. 3600 SOA ns4.nfon.net. hostmaster.nfon.net. 2026012701 3600 900 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0