Email SMTP Error Codes
Simple Mail Transfer Protocol (SMTP) is an internet and communication standard for sending emails using a plain text protocol. The sender specifies commands to the mail target or service and receives response codes. Below some of the common SMTP error codes that you may get when sending emails.
Note: SMTP error codes can be typically listed as three digits e.g. 523 or as a split notation 5.2.3 – those are the same SMTP error codes.
101 - Cannot Open Connection
Unable to connect – check your server host or port configuration
111 - Connection Refused
Connection refused by remote SMTP server
220 - Service is Running
Normally the first message you will get from the server indicating it is ready for SMTP commands
221 - Goodbye
The server is closing the connection – informative and not an error: all commands were successfully handled and the session is terminating.
250 - OK
Success reply – the mail server has successfully delivered the message!
354 - Start Mail Input
Missing mail data – typically identifies mail sender and recipient are OK but the actual mail content is missing. Remember to end with .
421 - Service Unavailable
Transient error – try sending again later. May also hint too many To/Cc/Bcc recipients.
422 - Recipient's Mailbox over Storage Limit
Recipient mailbox over storage limit or recipient mail server directory over storage limit
431 - Disk Full on Recipient's Server
Disk full or out of memory on the recipient’s server side.
441 - Recipient's Server not Responding
Recipient’s server is not responding – your server will automatically retry few times (per configuration).
442 - Connection Dropped
Your server started delivering the message but the connection was broken during the transmission. Typically an unusual transient error.
451- Request Action Aborted
Your action has been aborted, usually due to overloading at your ISP or your SMTP relay server.
450 - Requested Action not Taken
The server could not access the mailbox to deliver the message. Typically a transient error. Often followed by a second SMTP error code that has more specific context.
471 - Local Error
Local error at your mail server that usually requests to try again later, and comes in conjunction with a primary SMTP error (like 450 or 550).
500 / 501 - Syntax Error
Error in the command (500) or its arguments (501). In rare cases if repeated may indicate bad or intermittent connection or noisy line.
502 / 504 - Command not Implemented
The command is valid but has not been activated or supported by your specific server.
510 / 511 - Bad Email Address
Issued by the sending server. For an external address, it indicates misspelling, and for an internal one – the recipient does not exist in the mail system.
512 - Host for Recipient Domain not Found
One of the servers on the way to the destination is unable to resolve the domain name of a recipient’s email address.
523 - Message too Big
The Recipient’s mailbox cannot receive messages that big as it exceeds the size limit on the recipient’s end.
530 - Authentication Required
Originally this error indicated issue with the SMTP server on the sending side asking you to authenticate. Can now indicate STARTTLS require and/or your server blacklisted by the recipient’s one.
541 - Recipient Address Rejected - Access Denied
Typically anti-spam mechanism ir intrusion detection systems at the recipient’s end.
550 - Mailbox is Unavailable
A general error that is external to your own mail server and may include an additional refining error code.