_spf3.univention.de has no MX records.
Question and response
Question
dig _spf3.univention.de. MX
Answer
The response has no answer records.
Authority
univention.de. 300 SOA ns1115.ui-dns.biz. hostmaster.schlund.de. 2017081622 28800 7200 604800 300
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0