Changes since SOA version
IXFR is a zone transfer request that returns changes since a specific SOA version. Enter the SOA version to request changes since then.
Not an apex domain
The domain mx.core1.sfdc-ei37dk.mta.salesforce.com is not the root of its zone.
Instead, try zone transfer on sfdc-ei37dk.mta.salesforce.com .
Instead, try zone transfer on sfdc-ei37dk.mta.salesforce.com .