What does 554 mean?

SMTP 554 is a general transaction failure:

What it means:

The mail transaction has failed permanently. This is a catch-all code for various permanent errors, often policy-related.

Common causes:

Spam filtering: Message identified as spam. Policy rejection: Sender or content violates policy. Blocklist match: IP or domain blocklisted. Authentication failure: SPF, DKIM, or DMARC failures. Content filtering: Prohibited content detected.

Typical message examples:

"554 Message rejected" "554 5.7.1 Rejected as spam" "554 Transaction failed" "554 Denied by policy"

Response:

Read the full error message carefully. Address the underlying cause (reputation, content, authentication). May or may not require address suppression depending on cause.

554 is often the spam rejection code. Pay attention to the accompanying message for specifics.