Validate Email Addresses in CSV Files Online
Validate email addresses in CSV files directly in your browser. Add a boolean column flagging valid and invalid emails, or filter to show only invalid rows — no upload required.
How to validate Email Addresses in CSV files
- Drop your file onto the upload area. It is loaded into the in-browser engine and the first 200 rows are shown.
- Choose the email column. The tool preselects the first column whose name contains "email" or "mail", otherwise the first column.
- Pick an action: Flag all rows adds an email_valid column (true or false), Keep invalid only returns the rows that fail, and Keep valid only returns the rows that pass.
- Click Validate Emails. The counts of valid and invalid addresses and the percentage invalid appear next to the result.
- Download the result in the same format you uploaded.
Your file is processed locally in your browser and is never uploaded. The free limit is 50 MB per file; larger files work if your device has the memory for them.
Worked example
A conference exports its registration sheet before sending joining instructions. Some addresses were typed by hand at the desk, and one attendee left the field empty.
Input (CSV)
attendee,email,ticket
Priya Nair,priya.nair@uni.edu,student
Jonas Weber,jonas.weber@firma,standard
Sam O'Neill,sam.o'neill@mail.ie,standard
Lucía Gómez,lucia.gomez@correo.es,speaker
Wei Chen,,student
Tom Hart,TOM.HART@Example.CO.UK,standardSettings
- Email column: email
- Action: Flag all rows
Result
| attendee | ticket | email_valid | |
|---|---|---|---|
| Priya Nair | priya.nair@uni.edu | student | true |
| Jonas Weber | jonas.weber@firma | standard | false |
| Sam O'Neill | sam.o'neill@mail.ie | standard | false |
| Lucía Gómez | lucia.gomez@correo.es | speaker | true |
| Wei Chen | NULL | student | NULL |
| Tom Hart | TOM.HART@Example.CO.UK | standard | true |
Jonas's address has no dot and suffix after the domain, so it fails. Sam's address is legal under the email standard, but the apostrophe is outside the allowed character set, so it is flagged false. Upper case is fine. The empty cell gets no verdict. The summary reads 3 valid, 2 invalid (40.0% invalid), and Wei Chen is counted in neither.
Working with CSV files
The whole cell must match the pattern, so a stray space before or after an address makes it invalid. Hand-typed CSVs often have them, especially inside quoted fields. Run Trim Whitespace on the column first. A cell holding two addresses, such as "a@x.com; b@y.com", also fails. Split it with Split Column if you need to check each one.
Empty fields load as NULL. With Flag all rows they get an empty email_valid, and they are left out of both Keep invalid only and Keep valid only. Count blanks separately with Count By if you need them. Accented letters are rejected by the pattern even when the file is correct UTF-8. A file saved in another encoding may not load at all, so convert it with Convert Encoding first. The tool guesses the email column from a header containing "email" or "mail", so check its choice when a file has both a work_email and a personal_email column.
Frequently Asked Questions
Why is a CSV address that looks correct marked invalid?
Usually a leading or trailing space, a non-breaking space pasted from a web page, or a trailing semicolon. Trim the column and validate again.
Where do blank email cells in my CSV end up?
Nowhere in the filtered outputs. Blanks get an empty email_valid in Flag mode, and they are excluded from both Keep valid only and Keep invalid only.
What rule does the validator apply?
The whole value must be a local part of letters, digits and . _ % + -, then one @, then a domain of letters, digits, dots and hyphens ending in a dot and at least two letters. It is a practical format check, not a full RFC 5322 parser.
Can valid addresses be rejected, or bad ones accepted?
Yes, at the edges. Legal but unusual addresses fail, such as ones with an apostrophe, accented letters, quoted local parts or punycode endings like .xn--p1ai. A few malformed ones pass, such as those with two dots in a row. Review the invalid list before deleting anything.
Does it check that the mailbox exists?
No. Nothing is sent and no domain lookups are made. The check runs in your browser against the text only. Use a verification service if you need deliverability checks.
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