SMTP 5.7.1: delivery not authorized, message refused
RFC 3463policy_blockaction: review
5.7.1 is a policy rejection. The receiving server understood the message and refused it on purpose. The cause sits in the accompanying text, which is where the actual diagnosis lives: reputation, content, relaying, or authentication.
What it looks like in the log
550 5.7.1 Relay access denied
550 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using Spamhaus
550 5.7.1 Message rejected due to content restrictions
Why it happens
The receiver considers you unauthorised to relay through it.
A blocklist listing named in the response text.
Content or a URL in the message triggering a filter.
A recipient-side rule that has nothing to do with your reputation.
What to do
Read the text after the code. 5.7.1 on its own is not a diagnosis.
If a blocklist is named, work that listing specifically.
If it says relay access denied and you are the sender, check that you are connecting to the right host and authenticating.