Bounced emails are disappointing; it means some of your recipients are not receiving the email campaigns you’re sending them. And if the bounce rate is high, it will impact your deliverability. 

To minimize this problem, you should manage your bounces in a proactive way by applying some helpful tactics. So what must be done first?
The answer is: Identifying the bounce codes which are responses from the receiving mail server, specifying why the email was not successfully sent.

Code Summary Description
2.XXX.YYY Success Success specifies that the DSN is reporting a positive delivery action. Detail sub-codes may provide notification of transformations required for delivery.
4.XXX.YYY Persistent Transient Failure A persistent transient failure is one in which the message as sent is valid, but persistence of some temporary condition has caused abandonment or delay of attempts to send the message. If this code accompanies a delivery failure report, sending in the future *may be successful*. In this case, the bounce will be classified as a soft-bounce.
5.XXX.YYY Permanent Failure A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.

How does VBOUT handle bounce codes (messages)?

Once we get an email reply (bounce message) from the mail server, we use the numeric bounce code in order to identify if the bounce type is a soft bounce (temporary failure) or hard bounce (permanent failure). VBOUT will automatically unsubscribe email recipients who hard bounce. Also, if a recipient has more than one soft bounce, the system will consider him a hard bounce.

You can find below a list of two types of bounce codes. The 5.X Bounce Codes (Enhanced SMTP bounce responses) which are composed of 3 numbers separated by decimal points i.e. (5.1.0) and the traditional SMTP Status Codes which have 3-digit codes without decimal points i.e. (504). You may find both in the bounce reasons for a particular email campaign.

5.X Bounce Codes (Enhanced SMTP Bounce Responses - Permanent Failures)

Code Description
5.0.0 Address does not exist
5.1.0 Other address status
5.1.1 Bad destination mailbox address
5.1.2 Bad destination system address
5.1.3 Bad destination mailbox address syntax
5.1.4 Destination mailbox address ambiguous
5.1.5 Destination mailbox address valid
5.1.6 Mailbox has moved
5.1.7 Bad sender’s mailbox address syntax
5.1.8 Bad sender’s system address
5.2.0 Other or undefined mailbox status
5.2.1 Mailbox disabled, not accepting messages
5.2.2 Mailbox full
5.2.3 Message length exceeds administrative limit.
5.2.4 Mailing list expansion problem
5.3.0 Other or undefined mail system status
5.3.1 Mail system full
5.3.2 System not accepting network messages
5.3.3 System not capable of selected features
5.3.4 Message too big for system
5.4.0 Other or undefined network or routing status
5.4.1 No answer from host – the address is invalid
5.4.2 Bad connection
5.4.3 Routing server failure
5.4.4 Unable to route
5.4.5 Network congestion
5.4.6 Routing loop detected
5.4.7 Delivery time expired
5.5.0 Other or undefined protocol status
5.5.1 Invalid command
5.5.2 Syntax error
5.5.3 Too many recipients
5.5.4 Invalid command arguments
5.5.5 Wrong protocol version
5.6.0 Other or undefined media error
5.6.1 Media not supported
5.6.2 Conversion required and prohibited
5.6.3 Conversion required but not supported
5.6.4 Conversion with loss performed
5.6.5 Conversion failed
5.7.0 Other or undefined security status
5.7.1 Delivery not authorized, message refused
5.7.2 Mailing list expansion prohibited
5.7.3 Security conversion required but not possible
5.7.4 Security features not supported
5.7.5 Cryptographic failure
5.7.6 Cryptographic algorithm not supported
5.7.7 Message integrity failure

Traditional SMTP Status Codes

The table below is not a definitive list of all available status codes, only a representation of the most frequent ones.

Status Code Group Group Name Description
1xx Informational Request received, continuing process
2xx Success The action was successfully received, understood, and accepted
3xx Redirection Further action must be taken in order to complete the request
4xx Client Error The request contains bad syntax or cannot be fulfilled
5xx Server Error The server failed to fulfill an apparently valid request
Code Description
421 service not available, closing transmission channel
450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451 Requested action aborted: error in processing
452 Requested action not taken: insufficient system storage
500 The server could not recognize the command due to a syntax error.
501 A syntax error was encountered in command arguments.
502 This command is not implemented.
503 The server has encountered a bad sequence of commands.
504 A command parameter is not implemented.
550 User’s mailbox address is invalid
551 The recipient is not local to the server.
552 The action was aborted due to exceeded storage allocation.
553 The command was aborted because the mailbox name is invalid.
554 The transaction failed for some unstated reason.

Hope this list of codes will help you determine all the reasons to better reduce your email bounce rates.

Want to learn the best practices that improve your email deliverability? Check out this article.