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SMTP Response Codes

There are hundreds of SMTP response codes that can get thrown when something goes wrong, lets explore all the most common SMTP response codes and what they typically mean.

There’s nothing more frustrating than having a problem and not knowing how to fix it. The good news is that you don’t need to be a coding expert to understand SMTP response codes.

Ensuring your emails get to the desired recipients is an essential part of business. Being prepared to handle any SMTP error code that comes your way can help you run your business more efficiently.

This article will be your go-to guide for knowing what SMTP is, the structure of the response codes, the most common SMTP error codes, and what to do depending on the code you receive.

What Is SMTP?

Standard Mail Transfer Protocol, usually known as SMTP, is the system that sends your emails out to the server. Once in the server, Internet Message Access Protocol (IMAP) comes in to deliver the email to the person you’ve sent the message to.

Think of SMTP as your local post office and IMAP as your recipient’s mailperson. To ensure that SMTP can get your message into the hands of IMAP, you have to use the correct address. But sometimes, it can be a bit more complicated than that. 

When something goes wrong, you end up with a notification that doesn’t make much sense. (Until now, of course.) Understanding SMTP error codes can help your business reach more clients and know how to fix any problems that arise.

Error Code Structure

SMTP response codes come in the form of three-digit numbers that fall into five different response categories.

The first digit tells you which category the response is in.

  • 1XX: Informational – the server has accepted the request but hasn’t taken action yet

  • 2XX: Success – the server has completed the job

  • 3XX: Redirection – the server needs more information to complete the job

  • 4XX: Persistent Transient Failure – the server has a temporary failure, but the user should try again later

  • 5XX: Permanent Error – the server cannot fulfill the request


The second digit gives you some more information about why you received this response.

  • X0X: Syntax – syntax errors and commands that don’t fit any functional category

  • X1X:  Information – replies to requests for information like help or status update

  • X2X: Connections – issues within the transmission channel

  • X3X and X4X: Unspecified – uncategorized errors

  • X5X: Mail System – indicates the status of the receiver’s system

The third digit is an extra modifier, but there isn’t a clear set of rules for what each number represents in the different code combos. The server administrators determine what makes up this subclass if they need to add more information to an error code. The number can be any digit from 0-9. 

If an email is sent with no problems at all, the response code would be 250. In a perfect world, we would get no responses other than “250.”

SMTP Reply Codes

These reply codes are meant to show you the status of your message and help you understand why something succeeds or fails to go through.

This article will skip over the positive response codes simply because getting a positive response is the end of the interaction. Instead, this article will serve as a guide to help you identify and alter the errors that occur.

Common positive response codes include:

  • Code 200

  • Code 214

  • Code 220

  • Code 221

  • Code 250

  • Code 251

  • Code 252

Essentially all of these error codes indicate a message was successfully sent, and unless you are viewing live logs of an email server or looking at an email trace, you shouldn't ever see these error codes as a user.

SMTP Error Reply Codes

When you get an SMTP error code, it obviously means something has gone wrong. But understanding the triggers that lead to specific codes can help you correct the issue. 

Codes that begin with a four or a five indicate that the problem occurred during the delivery process. Additionally, these codes are used for any troubleshooting or communication problems.

Error Code
Error Message Definition
How To Fix
Code 421
<domain> service not available; the service isn’t currently available, and the connection will be closed; the mail server can’t make the connection now, but you may be able to send the email later.
This often occurs when the server is overwhelmed with a bunch of requests or with a generalized issue. Wait a bit, and then try to send the message again.
Code 450
User’s mailbox is unavailable; Requested action not taken – the user’s mailbox is unavailable; the recipient’s email has been locked, compromised, IP problems, or blocked.
The server will attempt to send the message again after a short amount of time. In the meantime, you can check that you’re operating on a reliable IP address.
Code 451
Local error in processing; server error; Your ISP’s server or the next server in line had a connection issue.
If you’re sending large emails in bulk through a free SMTP provider, you could run into this code a lot. Often, this error is the result of a message overload or an anti-spam filter. It’s best to check with your SMTP provider if you constantly get this error code.
Code 452
Too many emails or too many recipients; insufficient storage system; the server doesn’t have enough storage to send the message.
This occurs when your server gets overwhelmed. If you have too many recipients, then break your email list down into chunks. If you believe the issue is with storage, clear out the clutter in your inbox. Once you give your server some breathing room, it should be able to push the message through.
Code 455
Server unable to accommodate parameters; the server cannot complete the command at the moment.
Server unable to accommodate parameters; the server cannot complete the command at the moment.
Code 500
Syntax error; the server didn’t recognize the command; the command line is too long; the last command wasn’t recognized as a valid SMTP command.
This code is usually your server’s response to a bad interaction with your firewall or antivirus. Contact your firewall or antivirus provider for support to clear up the issue.
Code 501
A syntax error in parameters or arguments; the command from your server was correct, but the parameters (like the email address) were incorrect.
If you get a 501 error code, the first thing to do is make sure you entered the email address correctly. If there is no issue with the recipient’s email, check your internet connection and antivirus settings.
Code 502
The command cannot be implemented; the command from your server is valid but wasn’t activated.
This typically occurs when the receiving server does not accept the command. Contact your SMTP provider to resolve the issue.
Code 503
The server requires authentication; the server receives an incorrect sequence of commands.
Usually, a bad connection causes an incorrect sequence of commands from the server. If you receive a 503 message for “bad sequence,” check your connection and try again. If it’s an authentication issue, log out and log back in and resend the message.
Code 504
A command parameter is not implemented; the command and parameters are valid, but an additional action or parameter is needed.
Code 504 is similar to a 501 error as a syntax. Contact your provider for a solution.
Code 510/511
Bad email address
One of the email addresses in your TO, CC, BCC lines doesn’t exist. Make sure you have entered the addresses correctly and update any contact information to avoid these messages in the future.
Code 550
Non-existent email address on the remote side; user’s mailbox wasn’t found; the command was rejected for policy reasons; the server doesn’t have access to user’s mailbox
This error occurs when the email is typed in wrong, the recipient deactivates an old email account, or when the recipient’s firewall bounces it back. The best thing to do is double-check the entered address to ensure it’s correct. If there isn’t an issue with the address you have on file, contact the recipient through another means of communication and get their updated information.
Code 551
User not local or invalid address; relay denied; the recipient’s server is not the same as yours; the server will not forward.
This usually occurs when recipients are attempting to prevent spam. You can try <forward-path>, or you can contact your ISP provider and ask them to mark you as a certified sender.
Code 552
Exceeded storage allocation; the recipient’s mailbox is full; the message exceeds limit size.
If you’re sending an email with a lot of large attachments, try to eliminate some of the attachments or send multiple emails instead.
Code 553
Mailbox name invalid; incorrect email address; mailbox syntax incorrect.
Check the addresses in your TO, CC, and BCC lines and ensure the spelling is correct.
Code 554
The transaction has failed; permanent error; the server will not try to send the message again; no SMTP service.
The transaction has failed; permanent error; the server will not try to send the message again; no SMTP service.
Code 555
The server doesn’t recognize the email address format; MAIL FROM/RCPT TO parameters are not recognized or implemented.
Check the entry of the recipient’s email address and your connection.

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