spf.smtp.software has no MX records.
Question and response
Question
dig spf.smtp.software. MX
Answer
The response has no answer records.
Authority
smtp.software. 3600 SOA nsn1.mijndomein.nl. hostmaster.smtp.software. 2026022601 86400 3600 604800 3600
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0