Email bounce codes and rules

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A screen is full of computer code written in colors on a black background, visually representing email bounce codes and rules.

Email bounces and explanation of bounce codes

When you, as a customer at MarketingPlatform, send an email to a contact, a response will be sent back from the recipient’s mail server to MarketingPlatform’s mail server. If the email is bounced, there are a number of different codes that indicate why the email is bounced from the recipient’s mail server. From time to time, it may be problematic to accurately diagnose exactly why an email is bounced, as not all mail servers are equally good at returning a precise answer to why an email is bounced.

3 types of email bounce codes

There are 3 different types of email bounce codes: Hard, Soft or Other. Here follows the explanation:

  • Hard bounce: A hard bounce occurs when the incoming mail server at the recipient informs that the email address is not valid. Typically, this will mean that the user does not exist.
  • Soft bounce: A soft bounce is a condition where the email address of repeating soft bounces must be declared invalid. An example of this is a code 21 (see below). This can occur if the domain name no longer exists, or the DNS registration has expired or will be renewed tomorrow, or there was a temporary error in DNS lookup. If code 21 is continuous, the email address can be declared invalid.
  • Other: This is a state where reception fails, but here it is not the email address that causes a problem. The problem here can be spam blocking. Instead of completely removing the email address, you should instead investigate and resolve the spam issue, so that you can resend to the email address.

 

Below are the individual bounce error codes. See the description of the individual error codes below the table.

Code Description Type Example on SMTP response
                                   
0 Non-bounce Other This occurs when the bounce processor receives an email that it determines is not a bounce notification.
10 Bad email address Hard 550 User Unknown, Bad email addresses.
20 Temp failure - General Soft Temporary local problem - please try later.
21 Temp failure - DNS Failure Soft Remote host said: The email account that you tried to reach is disabled, Sorry, I couldn't find any host by that name. (#4.1.2), Domain or host resolution failure
22 Temp failure - Mailbox full Soft Remote host said: Recipient mailbox is full, user quota exceeded. 552 mailbox full
23 Temp failure - Message size too large Other Remote host said: The size of the message you are trying to send exceeds a temporary size limit of the server. 552 message exceeds size limits (server or policy limit).
29 Temp failure - Unable to connect Soft This is a soft bounce indicating that the sending server was unable to establish a connection with the recipient’s mail server. This can happen if the recipient domain does not exist, has DNS issues, is temporarily unavailable, or is blocking connections from the sending server. In some cases, corporate domains may also return this response for non-existent or deactivated employee email addresses, especially when the mail server is configured to reject unknown recipients during the SMTP connection process.
30 Bounce with no email address Other The ISP is determined that the message was a bounce, but it could not resolve the original email recipient email address.
40 General bounce Other This is an other bounce that could not otherwise be categorized.
50 Mail block - General Other This is an other bounce caused by the remote server rejecting the message for reasons that are not otherwise covered by a more specific “Mail block” category.
51 Mail block - Known Spammer Other This is an other bounce caused by the remote server rejecting the message because it considers the sender to be a spammer. These bounces are not specific to the individual message, but rather the sending server itself. This can be caused by listing on various public or private block lists.
52 Mail block - Spam detected Other The recipient's email server has blocked your email on the basis that the content resembles spam. This mail block is often triggered by something detected in your email content, but can also be your reply-to address or brand name that has a poor reputation. We treat this as a other bounce due to the fact that some mail servers and email providers respond with false or incorrect error codes.
53 Mail block - Attachment detected Other Attachment is detected in the email. This is an other bounce caused by the remote server rejecting the individual message due to its inclusion of one or more attachments.
54 Mail block - Relay denied Other This is an other bounce caused by the remote server refusing to serve as a relay for delivery of the message.
55 Mail block - SPF/DKIM/DMARC Other This is a bounce caused by the remote server refusing delivery of the message due to SPF, DKIM, or DMARC policies.
59 Mail block - Unable to connect Other Unable to connect or persistent delivery rejection. This is an other bounce where we were unable to connect to the remote server, however, this is to a large domain name, so we are reasonably confident that the domain name did not cease to exist, but that they are blocking all connections.
60 Auto-reply Other These are messages received by the bounce processor that were generated by an auto-reply system such as a vacation autoresponder. 
70 Transient bounce Other This is an other bounce caused by a temporary delay in message transmission. The remote server will usually continue trying to deliver the message for a limited period of time
90 Unsubscribe request Hard This is a hard bounce caused by a request to unsubscribe from the mailing list.
100 Challenge response Other This is an other bounce caused by the remote server issuing a challenge-response probe.
110 Over throttle limit Other This bounce happens when a message has been in the queue past the queue lifetime limit. It is scheduled for a delivery attempt. But that delivery attempt cannot be made because it would go over the configured throttle settings for that ISP or Relay Server.
120 Queue dumped Other This is a bounce where the system was told to dump the messages in the queue. All the messages dumped from the queue will bounce with this code.
121 Delivery paused then message expired Other This is a bounce where the server was told to pause delivery of messages in a queue. After the maximum retry time the messages are bounced, and they are bounced with this code.
200 Spam complaint N/A N/A