554 is a permanent negative SMTP reply, but providers use it for different rejection conditions.
Capture:
- complete reply;
- enhanced status code;
- remote hostname;
- sending IP;
- sender/recipient;
- timestamp;
- queue/message identifier.
Then classify the actual reason: policy, content, authentication, reputation, relay, recipient restrictions or another condition.
Do not make DNS changes based on the three-digit code alone.