If you send email regularly, you likely have received them — those notifications saying that the email you just sent was undeliverable. Trying to determine why your emails bounced back can be frustrating since the notifications usually contain error codes accompanied by cryptic error descriptions.

When you send an email, certain sets of rules, or protocols, are used to deliver it. The Simple Mail Transfer Protocol (SMTP) and Extended SMTP (ESMTP) are two commonly used protocols. Each one has its own set of codes that provide information about a variety of events related to the delivery of emails. The only time you will likely see the codes is if there is a problem.

SMTP Error Codes

When you send an email and there is a delivery problem, your email server will send you a notification about it. These notifications often include a three-digit SMTP error code in the format XXX.

If the error code starts with the number 4, there is a temporary problem delaying the delivery of your email. Your mail server will continue to try to deliver it. If you do not receive another message, the issue was likely resolved and your email was successfully delivered. However, if the mail server still cannot deliver the email after a certain amount of time, it will stop trying and notify you that it was undeliverable. The amount of time it keeps trying to send the email is dictated by your server administrator, but it is usually a few days.

If the error code starts with the number 5, it is a permanent, or fatal, error. In this case, your email did not reach its recipient, and the mail server will not try to redeliver it. Two of the most common SMTP error codes that email users receive are 550 and 552.

A 550 error code indicates that the email could not be delivered because the mailbox was not available. For example, you will get this code if you incorrectly typed an email address or if a mailbox no longer exists. You can even get this error code if your email is successfully delivered to the mailbox you specified, but the recipient has forwarded his or her mail to another mailbox and the forwarding process fails.

A 552 error code usually means an email was not delivered because of a mailbox quota or policy. A message might be too long in length or too large in size. You can also get this error code if the recipient’s mailbox is full.

As you can see, SMTP error codes only give you a general idea of why an email was undeliverable. Fortunately, ESMTP error codes can provide more information about the problem, assuming the bounce back notification includes them.

ESMTP Error Codes

Like SMTP codes, ESMTP error codes have three digits but they are separated with periods (X.X.X). Once again, an error code starting with the number 4 is a temporary problem and one that starts with 5 is a fatal error.

The Internet Assigned Numbers Authority publishes a table containing the ESMTP codes and their descriptions. The following list includes some of the common fatal ESMTP error codes in that table. If there is a fatal SMTP error code associated with the ESMTP code, it is listed in parentheses.

5.1.1 (550): The recipient’s mailbox address does not exist. For Internet email addresses, this means the address portion to the left of the @ is invalid.

5.1.2: The destination system specified in the email address does not exist or is incapable of accepting mail. For Internet email addresses, this means the address portion to the right of the @ is invalid.

5.1.6: The recipient’s mailbox address was valid at one time, but mail is no longer being accepted for that address.

5.1.8 (501): The sender’s system address does not exist or is incapable of accepting return mail. For domain names, this means the address portion to the right of the @ is invalid.

5.1.9: The recipient’s mailbox address is valid, but the message has been relayed to a system that does not use the same protocol.

5.2.1: The recipient’s mailbox exists, but is not accepting messages.

5.2.2 (552): The recipient’s mailbox is full, usually because the recipient has exceeded a per-mailbox quota set by the mail administrator. The recipient can delete messages to make more space available.

5.2.3 (552): The email’s length has exceeded a message length limit set by the mail administrator for each mailbox.

5.2.4: An email was sent to a mailing list (aka distribution group), but the list could not be expanded. Thus, the email could not be forwarded to the people on that list.

5.3.0 (550, 554): The destination system exists and normally accepts mail, but something about the system has caused the bounce back.

5.3.1: The destination system’s storage capacity has been exceeded. The recipient might not be able to delete messages to make room for additional ones.

5.3.2 (521): The destination system is not accepting messages. For example, the system might be undergoing maintenance.

5.3.3: Certain features in the message are not supported by the destination system.

5.3.4 (552, 554): The size of the message is larger than per-message size limit set by the mail administrator.

5.4.1: The outbound connection was not established because the remote system was either busy or unable to take a call.

5.4.2: The outbound connection was established but the message transaction was not completed because of a timeout or inadequate connection quality.

5.4.3 (550): The network system was unable to forward the message because a directory server was unavailable.

5.4.4: The mail system was unable to determine the next hop for the message because the necessary routing information was unavailable from the directory server.

5.4.5: The mail system was unable to deliver the message because it was congested.

5.4.6: A routing loop caused the message to be forwarded too many times because of incorrect routing tables or a user-forwarding loop. The latter occurs when a user forwards his or her mail to another email address and that email address is forwarded back to the user’s original address.

Your Error Code Not Listed?

If you have an ESMTP error code that is not shown here, you can find the complete list in the Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry. For a list of the SMTP error codes, check out the SMTP and ESMTP Error Code List. Your IT service provider can also answer any questions you might have about bounce backs.