spfdefault.mail.tue.nl has no MX records.
Question and response
Question
dig spfdefault.mail.tue.nl. MX
Answer
The response has no answer records.
Authority
mail.tue.nl. 900 SOA ipam.tue.nl. hostmaster.tue.nl. 1861 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0