SMTP Error Codes (Complete List and Solutions)

Error handling is one of the most important aspects of modern development. When developing applications with notification services, there is a high probability of getting SMTP-related errors.

Below is a comprehensive list of SMTP error codes. You'll find a short description for each in column #2 although if you'd like to dive deeper into what's causing that SMTP error and how to resolve it, click on the link icon in column #3.

CodeShort DescriptionDetails & Solutions
101Invalid SSL or TLS configuration
111Error connecting with the remote SMTP server
221Domain service closing transmission channel
250Requested mail action completed and OK
420Recipient's firewall/filter blocking mail
421Domain service not available, closing transmission channel
422The recipient has exceeded their mailbox limit
431Insufficient storage or out-of-memory issues
432Domain service not available, closing transmission channel
441Intermittent network connection, the server has not yet responded
442The server started to deliver the message but then the connection was broken
446Too many hops. Most likely, the message is looping
447Timeout occurred, check server connection
449A DNS problem occurred; check your smart host setting on the SMTP connector
450Requested mail action not taken: mailbox unavailable. request refused
451Requested action aborted: local error in processing Request is unable to be processed, try again
452Requested action not taken: insufficient system storage
453No mail
454TLS not available due to temporary reason. Encryption required for requested authentication mechanism
455Recipient's SMTP server cannot interpret parameters associated with the "MAIL FROM" or "RCPT TO" commands.
458Unable to queue messages for node
459Node not allowed: reason
471Anti-spam filter or firewall prevented message from sending
500Syntax error, command unrecognized
501Syntax error in parameters or arguments
502Command not implemented
503Bad sequence of commands
504Command parameter not implemented
510Check the recipient address
511Mailbox not found
512Domain can not be found. Unknown host.
513Relaying denied or Address type incorrect
515Destination mailbox address invalid
517Problem with senders mail attribute, check properties
521Domain does not accept mail
522Recipient has exceeded mailbox limit
523Server limit exceeded. Message too large
530Access Denied. Authentication required
531Mail system Full
533Remote server has insufficient disk space to hold email
534Authentication mechanism is too weak. Message too big
535Multiple servers using same IP. Required Authentication
538Encryption required for requested authentication mechanism
540Email address has no DNS Server
541No response from host
542Bad Connection
543Routing server failure. No available route
546Email looping
547Delivery time-out
550Requested action not taken: mailbox unavailable
551User not local; please try forward path
552Requested mail action aborted: exceeded storage allocation
553Requested action not taken: mailbox name not allowed
554Transaction failed
555Wrong protocol version
556Email sent is too large for the recipient's email server to process

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