About Bounce Codes

Unexpected issues can occur during the email delivery process. For example, the domain of the recipient can be unreachable or not exist at all. In this case, an error message or bounce is created by the Mail Transfer Agent that identifies the problem, and the bounce is returned to the sender of the message. The sender’s email address is called the From Address of the SMTP envelope.

Another problem might be that the domain has been found but the user doesn't exist on that domain. Again, a bounce is created and sent back to the sender of the original message. Both of these are examples of hard bounces. This means that not only was the email unable to be delivered but that it can never be delivered.

Another type of bounce is a soft bounce, which means that although the email couldn't be delivered now, but it might be possible to deliver the message in the future. Here are some bounce codes:

Bounce Code	Description
Address Moved	The recipient has changed the email address and the new address is available. This type of bounce occurs rarely.
Bad Sender	The email address is bad: either the address is formatted incorrectly (unlikely) or there's no such user.
Last Resort	The bounce parser couldn't determine the type of bounce, but it has the name of the original recipient.
Mailbox Problem	The email address exists, but the mailbox is full or temporarily locked. This type of bounce occurs rarely.
Message Too Large	The message size exceeds the recipient’s allowable limit.
Network Problem	Because of network problems, the MTA is unable to connect to either the receiving Mail Server or some other necessary server (such as a company’s LDAP server). This type of bounce occurs rarely.
Protocol Problem	The receiving Mail Server doesn't work with the version of SMTP the MTA uses. This type of bounce occurs rarely.
Security Problem	The receiving Mail Server doesn't allow email from your domain. This can occur when the receiving Mail Server blocks domains of known or suspected spammers.
System Problem	The receiving email server is having technical problems and can't accept mail right now.
Unparsable	The bounce parser couldn't classify the bounce message into any other category.

Note: If a recipient has activated a vacation mail response, then a vacation code is generated when an email is delivered. However, a vacation code is an auto-reply message code, not an error code.

For more information on bounce codes and the status of email addressees, see About Creating Classification Rules for Bounce Messages and Configuring the Status of Email Addresses.