spf.gwdg.de has no MX records.
Question and response
Question
dig spf.gwdg.de. MX
Answer
The response has no answer records.
Authority
gwdg.de. 60 SOA dns1.gwdg.de. hostmaster.gwdg.de. 679234870 21600 3600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0