SMTP troubleshooting
SMTP enhanced status codes, explained
Understand standards-based SMTP enhanced status codes and identify when DNS or sender authentication can be checked.
Use a provider-specific guide when the response came from Gmail, Outlook or Yahoo, because providers can assign narrower meanings to the same code.
Fix SMTP 5.4.3 directory server failureDiagnose SMTP 5.4.3 when a routing directory or DNS lookup cannot resolve the next mail destination.
Fix SMTP 5.4.4 unable to routeFix SMTP 5.4.4 by identifying the destination domain, routing table, MX target, connector, or smart host that cannot produce a next hop.
Fix SMTP 5.4.6 routing loop detectedFix SMTP 5.4.6 by tracing repeated Received hops, connectors, smart hosts, aliases, and hybrid mail routes.
SMTP 5.1.2: bad destination system addressDiagnose SMTP 5.1.2 when the recipient destination domain is invalid, misspelled, or has broken public DNS or mail routing.
SMTP 5.1.8: bad sender system addressDiagnose SMTP 5.1.8 when the sender's domain or system address cannot be resolved or accepted.
SMTP 5.2.2: recipient mailbox fullUnderstand SMTP 5.2.2 mailbox full errors, determine who owns the fix, and decide when a new delivery attempt is useful.
SMTP 5.3.4: message too largeFix SMTP 5.3.4 when a message exceeds the receiving system's size limit by reducing attachments or using a file link.
SMTP 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 5.7.21: no acceptable DKIM signatureUnderstand SMTP 5.7.21 and determine why a receiver accepted none of the message's DKIM signatures.
SMTP 5.7.22: no valid author-matched DKIM signatureUnderstand SMTP 5.7.22 when no valid author-matched DKIM signature aligns with the visible From domain.
SMTP 5.7.24: SPF validation errorDiagnose SMTP 5.7.24 SPF validation errors caused by DNS, syntax, dependencies, or temporary lookup failures.
SMTP 5.7.26: multiple authentication checks failedUnderstand SMTP 5.7.26 and separate SPF, DKIM, DMARC, and alignment failures before changing email DNS.
SMTP 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 5.7.23 : 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.
Primary sources: IANA SMTP enhanced status codes.