What are SMTP codes?

What are SMTP codes?

Deliverability FAQs

SMTP, also known as Simple Mail Transfer Protocol, refers to the internet standard for email transmission. When an email is sent, it is sent via these protocols in order for the most successful email deliverability possible. When emails are sent and are either delivered properly or hit an issue, there is one of many SMTP codes associated with the send. Here are the top 25 most common error SMTP codes you will encounter.

Common SMTP Codes


This indicates that a help message for a human will be following. Typical reasons for this code are a mail server status update or a reply to a system help request from a user.


This is a response to the "help" command. This code typically displays information about a server.


This is a typical first response you will receive from the server. This code means that the server is running and will normally contain a welcome message.


The server is ending the mail session; there is no more email send during this session.


The mail server has successfully delivered the message.252The recipient cannot be verified but the server attempts delivery anyway.


The server is ready to accept the message itself after having received the From and To information.


The service is not available and the connection will be closed because the server is unavailable because of a transient event. This means that the mail server isn't available now but may be available after.


The recipient's mailbox is full.


There is a server error on the recipient's side, which causes the command to be aborted.


There is not enough storage on the server.500The last command sent was not recognized as a valid SMTP or ESMPT command.


The last command was correct by the parameters (email address, for example) is incorrect.


The command from your server is valid but was not activated. This typically happens if the command is not supported by the receiving server.


There was an incorrect sequence of commands from your server.


The command and parameters are both valid but an additional action or parameter is missing.530Authentication is required on the recipient side, your server has been blacklisted by the recipient's server, or the recipient's mailbox does not exist.


Actions are not taken because the mailbox is unavailable.


The recipient has an invalid address. Or, if the sending address or recipient's address is not hosted on the local server, the message may not be relayed.


The server has exceeded its storage or the message exceeds limit size.


The name of the mailbox is invalid, the STMP server you are using does not have authentication/internet connection, or you are sending from an exchange server without public reverse DNS records pointing back to your server.


The transaction failed. Typically, this is due to a recipient server thinking your email is spam or your IP address is blacklisted.

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