custdkim.salesforce.com has no TXT records.
Question and response
Question
dig custdkim.salesforce.com. TXT
Answer
The response has no answer records.
Authority
custdkim.salesforce.com. 30 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2040819900 1800 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0