550 5.4.1 Recipient address rejected: Access denied

I recently implemented a DMARC policy. Everything seems to be as intended but I still get the occasional Delivery Status Notification (Failure) messages with the “Recipient address rejected” and I cannot find out what might cause it.
Any clue where I should look for a solution?

DMARC doesn’t force a recipient to accept email. It only lets them know when you would prefer that it be quarantined or rejected for impersonating your domain.

Sometimes the NDRs include an explanation. The Access denied message is a inconsistent with the ESMTP code of 5.4.1. An X.4.1 response is defined in RFC 3463 as No answer from host.

RFC 5248 indicates that ESMTP X.4.1 should be used in conjunction with an associated basic status code of 451, and makes no mention of 550 being applicable.

Due to the non-standard application of response codes, you will probably need to seek clarification from the recipient postmaster.

Thanks for this detailed answer!
A quick follow up question. If I understood you right this error is unrelated to my DMARC policy (the email would not have been delivered even in the absence of a DMARC policy, or with different settings).

Thanks again

Probably not, but since the use of the response codes is inconsistent with their established meaning, only the postmaster who configured the receiving mail server will know that answer with any degree of certainty.