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Uploading Files

The bulk email validator accepts .txt, .xls, .xlsx, and .csv file types. Please make sure your file is in one of our accepted formats. When you upload a file, our system will scan it. The file will be rejected if something unusual is detected. If you upload a .csv, .xls, or xlsx file, the first few columns will be displayed after you click "Validate File". Our system will try to determine the column's data type, as well. Any mistakes can be fixed before validation. If you can't get the system to accept your file, please use the help button on the bottom right to contact us.

Please make sure your file follows one of the formats here: Allowed File Formats


Column matching is important, because if you upload a .csv, .xls, or .xlsx file with a first name column, our system will use that information to tell you the gender of that email. Apart from the gender, our system also has the ability to determine the location, account creation date, first and last name of some of the emails you upload. It will use the data you provide and fill in the gaps. We don't store your emails or your data. Our Data Append feature uses public sources of information known to be 99% accurate. If the IP address is provided, we will append the city, region/state, zip code, and the country to the output.


All duplicate, uni-coded, and malformed email addresses will be removed from the list. So, if you upload a list of 5,000 emails and you have 1,000 duplicates, only 4,000 will be processed. No, you will NOT be charged for submitting duplicate, uni-code, or malformed emails. Invalid email syntax is different from uni-coded and malformed emails. Once your file is finished processing, there will be an arrow on the left of the file name that you upload, which will expand a section below to show you if we found any of the above mentioned conditions.


ZeroBounce will truncate all file names over 70 characters. We do this because windows will only support 160 characters in a file path and with log filenames customers hit that limitation when extracting the files from the zip file. Please name your files accordingly, to not exceed the 70 character limitation.


All files, whether it's one email or a million emails, will most likely take 20 minutes or longer to process. We can process the emails much faster, but we have an anti-greylisting technology which requires the results to be delayed by 15 minutes to reduce the number of "Unknown". We do have the ability to process 80-100 million emails a day. Some email domains take longer to validate than others, but you will not be waiting days for the results. It will take about 1 to 3 hours per 100,000 of mixed domains. The system will update the progress on every 100,000 validated emails. You can check the status on the "Download Your Results" page. Once your entire list is fully validated, you will receive an email notification letting you know your results are ready to be downloaded.

TIP: If you want faster results, break out the major ESP domains (Yahoo, Gmail, Hotmail, etc) from the others into separate files. These domains process much faster than the smaller ones.


Allowed file formats:

  • .txt
  • .csv
  • .xls
  • .xlsx

Examples of Accepted Text File Formats:

Example Text File 1:

A TXT file that is separated with Comma's

A TXT file that is separated with Comma's [ , ], Pipes [ | ], Tabs [ ], or Semi-Colons [ ; ] (without Carriage Returns and Line Feeds).

Example Text File 2:

A TXT File with only (Carriage Returns and Line Feeds)

A TXT File with only (Carriage Returns and Line Feeds).

Example Text File 3:

A TXT Files that looks like a CSV file will be converted to a CSV file when you upload it

A TXT Files that looks like a CSV file will be converted to a CSV file when you upload it.

Examples of Accepted CSV File Formats:

A CSV file is accepted if it's separated by Comma's [ , ], Pipes [ | ], Tabs [ ], or Semi-Colons [ ; ], or Carriage Returns/Line Feeds. If it's separated with a different delimiter, it will consider your CSV File as a single column file. If you save your spreadsheet as a CSV or TXT file, the system will accept it.

Example CSV File 1:

A typical accepted CSV file format

A typical accepted CSV file format

Example CSV File 2:

A typical accepted single column CSV file format

A typical accepted single column CSV file format

  • If your CSV file contains a header row (In the column matching screen) after you click "Validate File", please check the "Yes" for "Header Row" before clicking "Process File" , if you check "No" it will process the header row as an email address.
  • If your CSV file has multiple columns of email addresses, we will allow you to select which column of emails you want us to process
  • If we detect a large number of invalid emails or missing emails the system will reject it.
  • All rows that don't have an email address in it will be ignored and removed.
  • All duplicate emails will be removed, if a duplicate email is encountered the entire row is removed.
  • If you have multiple email addresses in the same cell, it will be processed as a single email address, which will all end up being invalid
  • We support up to 500 columns of data in your CSV file and a 1 gigabyte file size uploads.

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.
  • 98%+ 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 (98%+ 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