SMTP Status Codes and Meanings

There are many SMTP Status Codes for SMTP messages. These codes are very helpful in understanding the overall traffic of your message.

What is SMTP Status Code?

There can be occasions that your emails bounce back. When the email fails to deliver permanently, it will be sent back to the sender indicating it could not be delivered to the intended recipient. Usually, the reason why it was not sent will be included in the bounce back message. There will be also a SMTP Status Code. This status code is standard and all the status codes are predefined. To understand why your message failed to deliver, you could refer to the status codes and you can take necessary actions. The SMTP Status Codes are often called Bounce Codes.

Types of Bounce Codes

Did you that there exists different types of bounce codes? There are two types of bounce codes. They are listed below.

1) Traditional Bounce codes

2) Enhanced bounce codes

What is Traditional Bounce code?

The traditional bounce codes are a type of SMTP status codes. They are three digit codes. an example will be 550.

What is Enhanced bounce codes?

These are another type of SMTP status codes. In this method, the three digit codes are separated by decimal points. An example will be 5.5.0. It can be possible you will see both type of the bounce codes in a single bounce back email.

Traditional bounce codes and their meanings

421: <domain> Service not available, closing transmission channel

This means that the server (either your server or the server of the recipient end) is not available at the moment. It can be because the limit of simultaneous connections is exceeded.

450: Requested mail action not taken: mailbox unavailable

You will get this error when the mail could not be delivered because the mail box is busy or it is corrupted.

451: Requested action aborted: error in processing

This means that the server of your ISP or the first server that got your relay has a connections problem.

452: Requested action not taken: insufficient system storage

This error means that the email is not delivered due to insufficient storage space.

500: The server could not recognize the command due to a syntax error.

This error means that the server was unable to recognize a command. This can be caused by the interaction of an antivirus software.

501: A syntax error was encountered in command arguments.

It is another syntax error. But here, the problem is not in the command but in other parameters. Mostly, this will be due to the email address is invalid.

502: This command is not implemented.

As the message says, the particular command is not activated on your own server. You will need to contact the hosting provider to fix this issue.

503: The server has encountered a bad sequence of commands.

This error means that the server has pulled out the commands in a wrong order. This can be due to issues with connectivity. Sometimes it will be needed to authenticate again. You could do it with the username and password.

504: A command parameter is not implemented.

This is also a syntax error in the parameters. You need to contact WebHostingPeople for more details if you get this error.

550: User’s mailbox was unavailable

You will get this error if the mail box you are trying to send email to is not present on the recipient server.

551: The recipient is not local to the server.

You will get this error if your email account and the recipient email account are not locally hosted by the server.

552: The action was aborted due to exceeded storage allocation.

This error means that email you’ve sent has exceeded the incoming email size limit on the recipient server. You need to send a lighter message to get it delivered to the same user.

553: The command was aborted because the mailbox name is invalid.

This means the recipient address you have provided is invalid.

554: The transaction failed for some unstated reason.

This means that the email failed to deliver permanently. This happens when recipient server views your message as spam, or your server IP address is listed in an RBL.