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Email List Verification

SMTP BOUNCE CODES

SMTP Bounce codes are returned by the different mail servers indicating why they didn't accept the emails that you sent it. It can be for many different reasons, having an email bounce is not always an indicator of an email being invalid. It can be for many other reasons, the most common is that the spam filters identified the message as "Spam Content" and rejected it.

In order to determine why the emails that were sent bounced, it's important to have access to these codes, so you can make adjustments and decisions to remedy the issue.

Below you will find a summary of codes, keep in mind, there are thousands more, depending on the mail server the same codes will have different meanings, these are just a few examples.

Reference: RFC2821

5.X BOUNCE CODES (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

4.X BOUNCE CODES (TEMPORARY FAILURES)

  • 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