Other companies say it, we put it in writing. ZeroBounce, the leader in email validation, makes you the promise of cleaner and safer email lists. We guarantee the following accuracy rates at a minimum:

  • 96%+ Accuracy rates on AOL, Yahoo, and office 365 email accounts. This does not include disabled accounts.
  • 99% Accuracy rates on everything else.

How the Guarantee works:

If the accuracy rates are below these thresholds, we will refund 5 times the amount for each 1%. Let's say you have a 97% accuracy rate, which is 1% below the guaranteed threshold of (99% Accuracy rates on everything else). We will refund 5 x 1% (or 5%) of your total validation cost for those emails. If your accuracy rate is 2% below the guaranteed threshold, our refund will be of 10% of your costs. If it's 4% below, the refund will be 20%, and so on.

In order to qualify for the guarantee, the following conditions must be met:

  • You only email the _valid.csv and remove all toxic and disposable emails.
  • You email the list within 72 hours after cleaning.
  • Bounces must be invalid emails (Hard Bounces).
  • Bounces due to spam content, blacklisted IP's, technical Issues, invalid mail configuration, throttling, etc do not qualify for the guarantee.
  • The minimum list size must be 1000 emails.
  • This list must be an opt-in list that you collected and not purchased from a 3rd party
  • You must provide an export of your full mailing of all emails you sent to and their bounce responses (the actual SMTP Codes and messages of the bounces), not your email sender classifying it as a bounce. ZeroBounce does not guarantee the acceptance of your cleaned list at any ESP provider.


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


  • Code
  • 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


  • Code
  • 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