Understanding SMTP Codes and Troubleshooting Error Messages


Sometimes, the emails you send are not delivered to their intended recipient. In such cases, your email provider will send you a message notifying you of the error accompanied by a unique SMTP error code.

This article will introduce you to the unique codes for SMTP errors, their causes, and what you can do to rectify them.

What Is an SMTP Error Code?

First, SMTP refers to the Simple Mail Transfer Protocol, the most popular protocol for sending and receiving electronic mail. An SMTP error code is a unique three-digit code that identifies the cause of failure for a message sent using the protocol.

If you try to send an email using the SMTP protocol and it doesn’t get delivered, your email provider will send you an error code to help you identify the cause of the issue.

Five Classes of SMTP Response Status

Regular SMTP response codes are just 3-digit numbers. There are five types of possible responses after your email server issues an SMTP command:

  1. The server understood the command and implemented it successfully.
  2. It accepted the command but needs further information to process and complete it.
  3. It understood the command but took no further action.
  4. It has experienced a temporary failure. This can be rectified by issuing the commands repeatedly.
  5. It has experienced a permanent failure. This requires urgent attention from the host.

Positive SMTP Response Codes

Not all SMTP codes indicate failure. Some codes are positive, and they usually start with a 2 or 3.

200 Codes

  • 220 — The server is ready to perform its intended action.
  • 221 — The connection ends after the completion of all processes.
  • 250 — The requested action was completed and your message was delivered to the intended recipient.

300 Codes

SMTP codes that start with the number “3” show that the server understood the command but needs more information to complete it.

  • 334 — Waiting for the authentication credentials.
  • 354 — Server has received the “From” and “To” addresses and is waiting for the message body.

SMTP Response Error Codes

Failure codes start with a 4 or a 5. Codes starting with 4 indicate a transient failure, meaning that the message can be re-sent again at a later time. Those with a 5 indicate a permanent failure.

400 Codes

  • 421 — Service is unavailable, closing the connection.
  • 450 — The recipient’s mailbox is not available.
  • 451 — The recipient’s mailbox has exceeded its limits.
  • 452 — The server is overloaded and has insufficient storage space.
  • 455 — Unable to process the command.

500 Codes

  • 500 — Command is not recognized due to a syntax error.
  • 501 — A syntax error was encountered in the receiving address (invalid address or domain name).
  • 502 — Command not implemented.
  • 503 — The server received a bad sequence of commands.
  • 541 — Message rejected by the receiving server.
  • 550 — Recipient’s address unavailable (not found).
  • 551 — Recipient’s mailbox unavailable on the receiving mail server.
  • 552 — Not enough storage space on the receiving server.
  • 553 — Mailbox name does not exist.
  • 554 — Message rejected by the recipient’s mail server with no further explanation.

Possible Causes of SMTP Errors

There are many possible causes of such errors, including:

  • The message was recognized as spam and rejected.
  • The sender’s domain name or IP was identified as a spam source.
  • Spelling errors in the email address.
  • The recipient has exceeded the storage limit allocated by their email provider.
  • Temporary error with the mail servers.
  • An attachment in the email is too large.

How to Fix an SMTP Error?

Fixing the errors can be tough because there are quite a few possible causes, and you must first identify the source of the specific error. To complicate things even further, email providers are not always consistent in using SMTP error codes, with substantial differences even between major services. Doing the research can be difficult and time-consuming, but email service providers like UniOne will handle that for you and ensure that your emails are successfully delivered.


We have explained what SMTP error codes are and listed the common ones that you may encounter from time to time. Rectifying the errors can be strenuous, but you can offload that responsibility to a suitable email service provider like UniOne, which enables you to send transactional messages speedily and easily.




More like this

The Future of Chiropractic Medicine: Innovations, Trends, and Emerging Technologies Shaping Patient Care

Chiropractic medicine, rooted in the principles of holistic health...

Visit Vietnam’s Spectacular Lan Ha Bay

Vietnam is a Southeast Asian nation that is home...

Is Getting An eSIM For International Travel Worth It?

If you travel a lot frequently, you should think...

A Comprehensive Guide to Securing Wimbledon Tennis Tickets

The Wimbledon Championships is one of tennis's most prestigious...

Discover more from TotLol

Subscribe now to keep reading and get access to the full archive.

Continue reading