mx.core1.sfdc-ypmv18.mta.salesforce.com has no AAAA records.
Question and response
Question
dig mx.core1.sfdc-ypmv18.mta.salesforce.com. AAAA
Answer
The response has no answer records.
Authority
sfdc-ypmv18.mta.salesforce.com. 1800 SOA dns01.salesforce.com. hostmaster.salesforce.com. 2021011822 1800 900 2592000 1800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0