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 smtp-bep-stage-ap-southeast-1.gibmxdr.com is not the root of its zone.
Instead, try zone transfer on elb.ap-southeast-1.amazonaws.com .
Instead, try zone transfer on elb.ap-southeast-1.amazonaws.com .