SMTP Error Codes: What Do These Server Responses Mean?

January 25, 2023
Written by
Jesse Sumrak
Opinions expressed by Twilio contributors are their own

SMTP Error Codes: What Do These Server Responses Mean?

Not sure what an SMTP error code means? You're not alone. It can be shocking and confusing to see SMTP errors sometimes, and we want to eliminate the confusion with a quick-and-easy explanation.

Below, we'll explain everything you need to know about SMTP error codes, including short explanations of common ones you've likely seen before.

What is an SMTP error code?

Within the SMTP process, messages are relayed from one computer to another via SMTP servers. SMTP servers send response codes that indicate what has happened to messages.

While messages are usually sent without issue, different server response codes can indicate various things.

Below are a handful of SMTP server response codes that we’ve compiled that you might see when sending emails.

Common SMTP error codes

  • SMTP 250: This SMTP server response simply means everything went well and your message was delivered to the recipient server.
  • SMTP 421: Your message was temporarily deferred by the recipient server. This is usually a result of too many connections in a short timeframe or too many messages.
  • SMTP 450: Your message was not delivered because the other user mailbox was not available. This can happen if the mailbox is locked or is not routable.
  • SMTP 451: This response is sent when the message simply failed. Often times this is not caused by you, but rather because of a far-end server problem.
  • SMTP 452: This kind of response is sent back when there isn’t enough system storage to send the message. Your message is deferred until storage opens up and it can then be delivered.
  • SMTP550: The message has failed because the other user’s mailbox is unavailable or because the recipient server rejected your message.
  • SMTP 551: The mailbox your message was intended for does not exist on the recipient server.
  • SMTP 552: The mailbox your message was sent to does not have enough storage to accept your message.
  • SMTP 553: Your message was not delivered because the name of the mailbox you sent it to does not exist.
  • SMTP 554: This is a very vague message failure response that can refer to any number of problems either on your end or with the recipient server.

There are a variety of issues that can trigger these SMTP server responses, and you’re not expected to know how to handle all of them. SendGrid’s amazing support staff is here to help you figure out what went wrong and what you can do to correct it.

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To learn more about these and other SMTP server responses, read our Knowledge Base article. The article provides examples of all these responses as well as offers suggestions for correcting any problems.

Sending email involves multiple computers communicating with each other through a process call Simple Message Transfer Protocol (SMTP). SendGrid functions as an SMTP relay service to send billions of marketing and transactional emails every month.

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