US +1 650 600 3955 | EU +43 720 115 215

You’ve heard of the Napoleonic Code or the Da Vinci Code… but how important is a Bounce Code? Well, for e-mail marketing these codes are very important.

Essentially, a bounce is an e-mail that cannot be delivered. It has been rejected or bounced back to you from the e-mail server. Each rejection has a code associated with it – the bounce code.

Sometimes negative metrics can be as helpful as positive ones: Understanding bounce codes will allow your marketing campaigns to be more effective and give you an indication about the cleanliness of your data.

A Crash Course on Codes
E-mail servers communicate through different codes that mean the same thing to all servers. These codes are either three straight digits (123) called Simple Mail Transfer Protocol (SMTP) or digits separated by periods (1.2.3) called Extended SMTP (ESMTP).

7Sheep will report on SMTP codes, which are used more traditionally. We’re going to focus on these here.

Lower numbers in the 200 and 300 range tend to be less important for us because they mean things like:
252 – Cannot Verify user, will attempt delivery later
354 – Start message input and end with . (which simply indicates that the server is ready to accept the message)

For e-mail marketing purposes we need two sets of codes.

Soft Bounce – The 400s (Temporary Errors)
These errors mean that the system ran into some technical problem during delivery (server is down, inbox is full, message is too big, etc). This can happen due to delays or smaller issues occurring when the systems communicate with one another, but it doesn’t mean that there’s something wrong with your database. Bounce codes to look for are:

421/432 – Domain service not available, closing transmission channel
450 – Requested mail action not taken: mailbox unavailable. request refused
451 – Requested action aborted: local error in processing Request is unable to be processed, try again
452 – Requested action not taken: insufficient system storage
453 – No mail
454 – TLS not available due to temporary reason. Encryption required for requested authentication mechanism
458 – Unable to queue messages for node
459 – Node not allowed: reason

Hard Bounce – The 500s (Permanent Errors)
A hard bounce is a permanent fail. Of these, the most important and common one is the 550 bounce code. This one often comes when:

1. you’ve entered the email address incorrectly
2. the address doesn’t exist

If option 1 is the case, then you should check the entry and make sure that you have entered it correctly, otherwise it is best to remove the address from your list immediately because it no longer exists or it never existed.

Other hard bounce codes include:

500 – Syntax error, command unrecognized
501 – Syntax error in parameters or arguments
502 – Command not implemented
503 – Bad sequence of commands
504 – Command parameter not implemented
510 – Check the recipient address
512 – Domain can not be found. Unknown host.
515 – Destination mailbox address invalid
517 – Problem with senders mail attribute, check properties
521 – Domain does not accept mail
522 – Recipient has exceeded mailbox limit
523 – Server limit exceeded. Message too large
530 – Access Denied. Authentication required
531 – Mail system Full
533 – Remote server has insufficient disk space to hold email
534 – Authentication mechanism is too weak. Message too big
535 – Multiple servers using same IP. Required Authentication
538 – Encryption required for requested authentication mechanism
540 – Email address has no DNS Server
541 – No response from host
542 – Bad Connection
543 – Routing server failure. No available route
546 – Email looping
547 – Delivery time-out
550 – Requested action not taken: mailbox unavailable
551 – User not local; please try forward path
552 – Requested mail action aborted: exceeded storage allocation
553 – Requested action not taken: mailbox name not allowed
554 – Transaction failed