571 Delivery not authorized, message refused, PRT problem

571 Delivery not authorized, message refused, PRT problem

I'm working for a school that uses the domain @piancaschool.it with zoho.eu.
Pianca School is a private school in Italy.
Several recipients are rejecting our emails that uses this domain.
From what I could understand this error is generated by Exchange servers:  https://bobcares.com/blog/how-to-fix-571-delivery-not-authorized-message-refused/
We checked and piancaschool.it is not in any blacklist.
We contacted, by other means, the recipients that are rejecting our mails and asked them if they are actively (unknowingly, as we are talking about the children's parent's email addresses) blocking piancaschool.it. They gave us negative responses. In fact, they also offered their help to figure out the issue, but so far we only suspect that this issue is caused by missing PTR (reverse DNS) records for piancaschool.it.
The school also sends some emails from Google's SMTP always with @piancaschool.it (importing the mails from zoho with POP3 and then "sending as"). However I personally tried to send emails directly from the zoho portal and the problem was constantly there regardless.
Our DKIM and SPF record seems to be fine as Zoho portal and other online validation tools successfully evaluated them.