mx6.mta.tawk.to has no CNAME records.
Question and response
Question
dig mx6.mta.tawk.to. NS
Answer
The response has no answer records.
Authority
tawk.to. 801 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2397581131 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0