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 un38b4jo6kcgv9dp0flqc9afop6ud28k.smtp.dk is not the root of its zone.
Instead, try zone transfer on smtp.dk .
Instead, try zone transfer on smtp.dk .