custdkim.salesforce.com has no MX records.
Question and response
Question
dig custdkim.salesforce.com. MX
Answer
The response has no answer records.
Authority
custdkim.salesforce.com. 30 SOA mastername.salesforce.com. hostmaster.salesforce.com. 2040765481 1800 900 2592000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0