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 front2v2.siemens.flowmailer.net is not the root of its zone.
Instead, try zone transfer on siemens.flowmailer.net .
Instead, try zone transfer on siemens.flowmailer.net .