554 5.7.1 is the same policy refusal as 550 5.7.1, riding on a different base SMTP reply. RFC 3463 defines 5.7.1 identically regardless of which base code carries it: "delivery not authorized, message refused." The receiving server decided this message is not getting through, for a policy or authentication reason rather than a technical one.
What changes is the base code's own meaning. RFC 5321 defines 554 as "Transaction failed," noting that for a DATA command specifically this can mean the server does not accept mail from your source at all, rather than one address on that message being refused.
A 554 more often signals a whole-transaction refusal, sometimes issued before the receiving server has even processed individual recipients, where a 550 typically refuses one specific recipient after accepting the message for processing. That is a difference in where in the SMTP conversation the refusal happened, not a difference in cause.
It is tempting to read 554 as a more serious signal than 550 and jump straight to assuming a reputation block. The enhanced code is the actual diagnosis in both cases, and authentication is still the faster, cheaper thing to rule out first, before spending time reading Postmaster data.
Not in a way that changes the diagnosis. Both are permanent policy refusals per RFC 3463's 5.7.1 definition. 554 additionally signals the whole transaction failed rather than one recipient being refused, which matters for how you read the log, not for which cause you check first.
RFC 5321 defines 554 as a transaction failure, which for a DATA command specifically can mean the server does not accept mail from your source at all, a step earlier than a per-recipient refusal like a typical 550.
No. The enhanced code, 5.7.1, is what names the cause: authentication, reputation, or an explicit block. The base code changes what the receiving server did procedurally, not which cause you check first.