ns.zixcorp.com has no MX records.
Question and response
Question
dig ns.zixcorp.com. MX
Answer
The response has no answer records.
Authority
zixcorp.com. 300 SOA ns01.zixcorp.com. noc.zixcorp.com. 2025102401 3600 300 2419200 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0