Email troubleshooting guides
Generic delivery errors, organized by problem
Troubleshoot common delivery failures that are not limited to one receiving provider.
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SMTP 4.2.2: Recipient mailbox fullSMTP 4.2.2 means the recipient mailbox is over its storage quota; keep normal retries and ask the recipient or mailbox administrator to free space.
SMTP 4.3.0: Temporary mail system problemSMTP 4.3.0 is a temporary, otherwise unspecified mail-system failure; preserve retries and investigate the server named in the full response if it persists.
SMTP 4.4.1: Destination host did not answerSMTP 4.4.1 means the next-hop mail host did not answer; keep retries and check the destination MX hosts, port 25 reachability, and firewall path.
SMTP 4.4.2: Connection timed outSMTP 4.4.2 means an SMTP connection was established but failed or timed out before the message transaction completed.
SMTP 4.4.3: DNS or directory server failureSMTP 4.4.3 means a temporary DNS or directory lookup failure prevented routing; identify the queried name and correct timeout, SERVFAIL, DNSSEC, or resolver problems.
SMTP 4.4.4: Unable to route the messageSMTP 4.4.4 means the sender could not determine a next hop; verify the destination domain and its usable MX or fallback address route.
SMTP 4.4.5: Mail system congestionSMTP 4.4.5 means a mail system is temporarily congested; preserve queued retries and reduce pressure while its operator restores capacity.
SMTP 4.5.3: Too many recipientsSMTP 4.5.3 means the transaction contains too many recipients; split the recipient list into smaller SMTP transactions and retry responsibly.
SMTP 4.7.0: Temporary policy or security failureSMTP 4.7.0 is an unspecified temporary security or policy failure; use the exact response text to select the right fix while preserving normal retries.
SMTP 4.7.1: Temporary authorization or policy rejectionSMTP 4.7.1 is commonly used for a temporary authorization or policy rejection; preserve retries and follow the receiver-specific text.
SMTP 5.1.1: Recipient mailbox does not existSMTP 5.1.1 means the destination mailbox does not exist; confirm the address, remove stale suggestions, and stop retrying a permanent bounce.
SMTP 5.1.3: Invalid recipient address syntaxSMTP 5.1.3 means the recipient address syntax is invalid; correct the exact address generated by the sender before retrying.
SMTP 5.1.6: Mailbox moved without forwarding addressSMTP 5.1.6 means a formerly valid mailbox moved without a forwarding address; obtain the new address and stop retrying the old one.
SMTP 5.1.7: Invalid sender address syntaxSMTP 5.1.7 means the sender mailbox address syntax is invalid; correct the envelope sender or message sender field generated by the application.
SMTP 5.2.1: Mailbox disabledSMTP 5.2.1 means the recipient mailbox is disabled and not accepting mail; confirm its status or use a different valid address.
SMTP 5.2.3: Message too large for the mailboxSMTP 5.2.3 means the message exceeds a recipient mailbox size limit; reduce the complete encoded message or use an approved file-sharing link.
SMTP 5.6.0: Message format failureSMTP 5.6.0 is an unspecified permanent message-content or media failure; inspect the exact response and raw MIME source before rebuilding the message.
SMTP 5.7.0: Generic security or policy rejectionSMTP 5.7.0 is an unspecified permanent security or policy rejection; use the full receiver text to identify the policy before changing anything.
SMTP 5.7.1: Delivery not authorizedSMTP 5.7.1 means delivery is not authorized; use the full response to identify a recipient restriction, relay permission, transport rule, or filtering policy.
Primary sources: Primary technical reference.