spf.srv2.de has no NS records.
NS records for this zone are defined at srv2.de.
NS records for this zone are defined at srv2.de.
Question and response
Question
dig spf.srv2.de. NS
Answer
The response has no answer records.
Authority
srv2.de. 2560 SOA a.ns.optivo.de. hostmaster.srv2.de. 1489563392 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0