SMTP Error Codes

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.

SMTP 101

101 - Cannot Open Connection

Unable to connect – check your server host or port configuration

SMTP 111

111 - Connection Refused

Connection refused by remote SMTP server

SMTP 220

220 - Service is Running

Normally the first message you will get from the server indicating it is ready for SMTP commands

SMTP 221

221 - Goodbye

The server is closing the connection – informative and not an error: all commands were successfully handled and the session is terminating.

SMTP 250

250 - OK

Success reply – the mail server has successfully delivered the message!

SMTP 354

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 .

SMTP 421

421 - Service Unavailable

Transient error – try sending again later. May also hint too many To/Cc/Bcc recipients.

SMTP 422

422 - Recipient's Mailbox over Storage Limit

Recipient mailbox over storage limit or recipient mail server directory over storage limit

SMTP 431

431 - Disk Full on Recipient's Server

Disk full or out of memory on the recipient’s server side.

SMTP 441

441 - Recipient's Server not Responding

Recipient’s server is not responding – your server will automatically retry few times (per configuration).

SMTP 442

442 - Connection Dropped

Your server started delivering the message but the connection was broken during the transmission. Typically an unusual transient error.

SMTP 451

451- Request Action Aborted

Your action has been aborted, usually due to overloading at your ISP or your SMTP relay server.

SMTP 450

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.

SMTP 471

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).

SMTP 500/1

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.

SMTP 502/4

502 / 504 - Command not Implemented

The command is valid but has not been activated or supported by your specific server.

SMTP 510/11

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.

SMTP 512

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.

SMTP 523

523 - Message too Big

The Recipient’s mailbox cannot receive messages that big as it exceeds the size limit on the recipient’s end.

SMTP 530

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.

SMTP 541

541 - Recipient Address Rejected - Access Denied

Typically anti-spam mechanism ir intrusion detection systems at the recipient’s end.

SMTP 550

550 - Mailbox is Unavailable

A general error that is external to your own mail server and may include an additional refining error code.

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