dns06.salesforce.com has no TXT records.
Question and response
Question
dig dns06.salesforce.com. TXT
Answer
The response has no answer records.
Authority
salesforce.com. 1800 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2016655889 600 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0