spf.signet.nl has no MX records.
Question and response
Question
dig spf.signet.nl. MX
Answer
The response has no answer records.
Authority
signet.nl. 3600 SOA nsauth1.signet.nl. hostmaster.signet.nl. 2025103000 28800 7200 1814400 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0