Fpo9.mail.dk #5.7.1 smtp; 550 5.7.1 rejected by DMARC

My client uses an external provider to send email on my clients behalf - just like newsletter or other service.
it works great most of the time but since a week now one specific receiver are blocking with this message: fpo9.mail.dk #5.7.1 smtp; 550 5.7.1 rejected by DMARC
unfortunately this is a rather large tele company so my client experince this with more than one receiving address. the domains send to are @os.dk and @sport.dk which this tele compnay apparently hosts.
My client has dmarc and spf in place and the “newsletter service” is in the spf. any suggestions to go here ?
thank you

Has your client configured the “external provider’s newsletter service” correctly, so newsletters are DKIM signed using the RFC5322 From domain? Has the configuration been verified lately?

Also, as you may remember, many newsletter services use a custom return path that differs from the sender domain in order to manage bounces, so the newsletter service should be in the SPF for the custom return path domain.

Finally, mail forwarding on the receiver’s end will typically break DMARC SPF alignment, making DKIM the only option for DMARC compliance.