Messages summary code meanings

Simple Mail Transfer Protocol (SMTP) is the communication protocol for sending email. Each time you make an SMTP call, it returns a response in the form of a three-digit code. This code is included in the “Summary” column of the Activity Log.

SMTP codes as part of activity log summary

The first digit of the code tells you what kind of response it is. For example, 200 responses are usually success responses, 400 responses are usually deferrals, and 500 responses are hard failures that are not retried.

The most common code you’ll see is 250, which means the message was accepted by the recipient server and your send was successful.

Following are some other codes you might see.

  • 421: Messages are temporarily deferred because of recipient server policy. Often, this is because too many messages or connections have been made in too short a time.

  • 450: The message failed because the recipient's mailbox was unavailable, perhaps because it was locked or was not routable at the time.

  • 451: The message simply failed, usually due to a far-end server error.

  • 452: The message has been deferred due to insufficient system storage.

  • 550: The user’s mailbox was unavailable, usually because it could not be found, or because of incoming policy reasons. It is likely a fake, or it was mistyped.

  • 551: The intended mailbox does not exist on this recipient server.

  • 552: The recipient’s mailbox has exceeded its storage limits. This is usually a sign this is an abandoned email.

  • 553: The message was refused because the mailbox name is either malformed or does not exist.

  • 554: This is a default response that can be caused by a lot of issues. There is usually an accompanying message that will tell you more about the response.

  • 602: The message cannot be delivered to the recipient after attempting to send for eight hours.

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