Why was this mail rejected (HELO - SPF_Not_Pass)?

All mails from this particular source have been rejected at the HELO check despite appearing to have come from an authorised origin. I've had to disable SPF checking since I can no longer trust it.

May 14 02:35:24 calnan postfix/smtpd[19581]: connect from Mailer1.co-op.co.uk[194.28.124.70]
May 14 02:35:25 calnan postfix/smtpd[19581]: Trusted TLS connection established from mailer1.co-op.co.uk[194.28.124.70]: TLSv1 with cipher RC4-SHA (128/128 bits)
May 14 02:35:26 calnan policyd-spf[19587]: Read line: "client_name=Mailer1.co-op.co.uk"
May 14 02:35:26 calnan policyd-spf[19587]: Read line: "reverse_client_name=Mailer1.co-op.co.uk"
May 14 02:35:26 calnan policyd-spf[19587]: Read line: "helo_name=Mailer1.co-op.co.uk"
May 14 02:35:26 calnan policyd-spf[19587]: Read line: "ccert_subject=co-op.co.uk"
May 14 02:35:26 calnan policyd-spf[19587]: Fail; identity=helo; client-ip=194.28.124.70; helo=mailer1.co-op.co.uk; <email address hidden>; receiver=<user>@<domain>
May 14 02:35:26 calnan policyd-spf[19587]: Action: reject: Text: Message rejected due to: SPF fail - not authorized. Please see http://www.openspf.net/Why?s=helo;id=mailer1.co-op.co.uk;ip=194.28.124.70;r=<user>@<domain>
May 14 02:35:26 calnan postfix/smtpd[19581]: NOQUEUE: reject: RCPT from Mailer1.co-op.co.uk[194.28.124.70]: 550 5.7.1 <<user>@<domain>>: Recipient address rejected: Message rejected due to: SPF fail - not authorized. Please see http://www.openspf.net/Why?s=helo;id=mailer1.co-op.co.uk;ip=194.28.124.70;r=<user>@<domain>; from=<email address hidden> to=<<user>@<domain>> proto=ESMTP helo=<Mailer1.co-op.co.uk>