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 v5206.v508e7d51.usw1.send.mailgun.net is not the root of its zone.
Instead, try zone transfer on v508e7d51.usw1.send.mailgun.net .
Instead, try zone transfer on v508e7d51.usw1.send.mailgun.net .