mx.core1.sfdc-ei37dk.mta.salesforce.com has no MX records.
Question and response
Question
dig mx.core1.sfdc-ei37dk.mta.salesforce.com. MX
Answer
The response has no answer records.
Authority
sfdc-ei37dk.mta.salesforce.com. 10 SOA dns01.salesforce.com. hostmaster.salesforce.com. 2023050301 1800 900 2592000 10
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0