A 550 5.7.1 generally points toward a policy or security rejection, but the diagnostic text is what makes the case actionable.
Capture the evidence
Keep the complete response, remote hostname, sender, recipient, sending IP, timestamp and queue/message ID.
Then classify it
Look for wording related to:
- authentication or DMARC;
- relay authorization;
- spam or reputation;
- tenant or organizational policy;
- prohibited content;
- blocklists;
- sender restrictions.
Do not immediately change SPF or ask for a blocklist removal just because the code contains 5.7.1.