mail.smtp18.com has no TXT records.
Question and response
Question
dig mail.smtp18.com. TXT
Answer
The response has no answer records.
Authority
smtp18.com. 1800 SOA amy.ns.cloudflare.com. dns.cloudflare.com. 2397204337 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0