Email troubleshooting guides
Generic SMTP errors, organized by problem
Understand standards-based SMTP enhanced status codes and the evidence needed to resolve them.
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SMTP 451 or 550 5.7.24: SPF validation errorDiagnose SMTP 5.7.24 SPF validation errors caused by DNS, syntax, dependencies, or temporary lookup failures.
SMTP 5.1.8: Bad sender's system addressDiagnose SMTP 5.1.8 when the sender's domain or system address cannot be resolved or accepted.
SMTP 5.3.4: Message too big for systemFix SMTP 5.3.4 when a message exceeds the receiving system's size limit by reducing attachments or using a file link.
SMTP 5.4.3: Directory server failureDiagnose SMTP 5.4.3 when a routing directory or DNS lookup cannot resolve the next mail destination.
SMTP 550 5.7.20: No passing DKIM signature foundUnderstand SMTP 5.7.20, inspect the DKIM selector and public key, and fix a missing or failing DKIM signature without guessing DNS values.
SMTP 550 5.7.22: No valid author-matched DKIM signature foundFix SMTP 550 5.7.22 when DKIM passes, but no valid signature identifies the author in the From header.
SMTP 550 5.7.23: SPF validation failedUnderstand enhanced SMTP status 5.7.23 SPF validation failed, identify the sender IP and MAIL FROM domain, and check the relevant SPF configuration.
SMTP 550 5.7.26: Multiple authentication checks failedUnderstand SMTP 5.7.26 and separate SPF, DKIM, DMARC, and alignment failures before changing email DNS.
SMTP 550 5.7.27: Sender address has null MXFix SMTP 5.7.27 by checking whether the sender domain intentionally publishes Null MX and cannot receive delivery status notifications.
SMTP 5.4.4: Unable to routeFix SMTP 5.4.4 by checking the recipient domain and the route, connector, transport map, or smart host on the server that generated the error.
SMTP 5.4.6: Routing loop detectedFix SMTP 5.4.6 by finding the repeated Received hops and correcting the connector, smart host, alias, or hybrid route that sends mail back.
SMTP 550 5.1.2: Bad destination system addressFix SMTP 550 5.1.2 by checking the rejected recipient address and the recipient domain's DNS and mail route.
SMTP 550 5.7.21: No acceptable DKIM signature foundResolve SMTP 550 5.7.21 by identifying why the receiver rejected every passing DKIM signature as unacceptable.
SMTP 552 5.2.2: Mailbox fullResolve SMTP 552 5.2.2 by asking the recipient to free mailbox storage or have their administrator raise the quota.
Primary sources: Primary technical reference.