Why is the message blocked due to SPF?

If an email has been rejected due to SPF (sender policy framework) this is because the sender is sending from a location that is not in the SPF record of the sending domain. An SPF record outlines the host names and IP addresses that the sender can send from. eg:

 

smxemail.com txt
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 14458
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;smxemail.com. IN TXT

;; ANSWER SECTION:
smxemail.com. 7200 IN TXT "v=spf1 include:spf.nz.smxemail.com -all"

If the SPF record is set to hard fail (-all at the end of the record) and the email is coming from a location which is not in the SPF record then the SMX platform will reject the message with a bounce-back to senders mail server.

 

A handy site to check an SPF record is https://mxtoolbox.com/spf.aspx