Bulk email syntax checker: find the badly formatted addresses
A bulk email syntax checker tests how each address is written, not whether its mailbox exists. Paste a list or upload a CSV and every address is checked against the RFC 5321 and 5322 format rules; each failure names the rule it breaks, and common domain typos get a suggested fix. It runs in your browser, with no row limit.
Syntax only. This checks how each address is written; it does not contact any mail server and cannot tell whether a mailbox exists.
Sample result
jane..doe@acme.example is invalid: it has two dots in a row before the @.
- Address
- jane..doe@acme.example
- Issue
- Two dots in a row before the @
- Rule
- RFC 5322 §3.4.1
What this means for you
Fix or remove it; mail to a badly formatted address never arrives.
- The exact format error on every address
- Names the RFC rule each one breaks
- Suggests a fix for typo domains
- Download a full report or valid rows only
- RFC 5322
- Bulk
- Typos
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
How do I check email address format in bulk?
- Paste one address per line, or upload a CSV. In a CSV the email column is found automatically. Files can be up to 25 MB.
- Press Check syntax. Large files are processed in chunks, so 50,000 rows take a few seconds without freezing the page.
- Read the problem rows: each shows what is wrong and the rule behind it. Rows marked Check are typos or unusual forms worth a look rather than certain errors.
- Download the full report, or only the valid rows, and fix or drop the rest.
Example
- You enter
- Two addresses:
jane..doe@acme.exampleandsam@gmial.com. - You get
jane..doe@acme.exampleis invalid because of two dots in a row before the @ (RFC 5322 §3.4.1).sam@gmial.comhas valid syntax but is flagged as a likely typo, withsam@gmail.comsuggested.- Limits
- Checks format only, no mail server is contacted. Runs in your browser with no row cap; an uploaded file can be up to 25 MB.
What is valid email syntax?
Valid email syntax means a local part, one @ and a domain, each written within the limits the RFCs set. The part before the @ can be at most 64 characters and the whole address at most 254 (RFC 5321). It may contain letters, digits, dots and the symbols ! # $ % & ' * + / = ? ^ _ { | } ~ - (and the backtick), but not two dots in a row and not a dot at either end (RFC 5322).
The domain must have at least one dot, each part (label) must be 1 to 63 characters of letters, digits and hyphens, no label may start or end with a hyphen, and the top-level domain must be at least two letters (RFC 1035, RFC 1123).
This checker reports which of those rules an address breaks: a missing or doubled @, a space, a doubled or edge dot, a local part over 64 characters, a bad domain label or a missing top-level domain. Common slips on big webmail domains, such as gmial.com or hotmial.com, and endings like .con or .cmo are flagged as Check with the domain that was probably meant.
Two forms are technically allowed but flagged as Check rather than passed: quoted local parts such as "john smith"@example.com, and IP-address domains such as user@[192.0.2.1]. RFC 5322 permits both, but almost no signup form, CRM or sending platform accepts them, and on a marketing list they are nearly always junk. Internationalised addresses with non-ASCII characters (RFC 6531) are flagged the same way, because many sending systems still can't deliver to them.
How is this different from the list health check?
This page does one job, format, in more depth: it has no row limit, makes no network requests at all, and tells you exactly why each address fails. Our list health check gives a quick overview of a list of up to 5,000 rows instead: duplicates, syntax, typo domains, disposable and role addresses, and domains that can't receive mail. Use this one when you are fixing a form, an import mapping or a data pipeline rather than just cleaning a list.
Does a valid email format mean the address exists?
No. jane@northwind-example.com passes every rule above whether or not Jane works there, whether the domain has a mail server, or whether the company shut down last year. Syntax is the cheapest filter and the first one to run, but it only catches typing and data errors. It catches none of the dead mailboxes that make up most hard bounces.
The second step is checking that the domain can receive mail at all, which the MX lookup does for one domain. The third is asking the mail server whether the specific mailbox exists, which is the SMTP check that separates a list that looks clean from one that delivers. The difference is laid out in syntax check vs SMTP verification.
Syntax check, MX lookup or SMTP check: which do I need?
Each step catches a different kind of bad address. Run them in this order:
| Syntax check | MX lookup | SMTP mailbox check | |
|---|---|---|---|
| What it tests | How the address is written | Whether the domain can receive mail | Whether the mailbox exists |
Catches jane@@acme.com | Yes | No | Yes |
| Catches a domain with no mail server | No | Yes | Yes |
| Catches a closed mailbox | No | No | Yes |
| Network requests | None | DNS only | Talks to the mail server |
| Where | This page | MX lookup | Verify email |
Who uses this tool, and for what?
- SaaS teams
- Run a signup table through it to see which malformed addresses your form let in, and which rule your front-end validation is missing.
- Sales ops
- Check a CSV before a CRM import, so rows with two @ signs, spaces or a missing domain are fixed instead of failing the import or creating junk contacts.
- Marketers
- Find the
gmial.comand.contypos in an event or lead-magnet list and fix them before the first send, then remove duplicates. - Cold emailers
- Strip badly formatted rows out of a scraped list first, so you only pay to verify addresses that could exist.
What can’t this tool tell you?
- Syntax only. An address can pass every rule and still belong to a closed mailbox or a domain with no mail server.
- Any top-level domain of two or more letters passes; it doesn't check the ending against the list of real TLDs, apart from common slips like
.con. - Typo suggestions cover the big webmail domains only.
jane@acmee.comforacme.comlooks valid. - Quoted local parts, IP-address domains and non-ASCII addresses are marked Check, not passed, even though the RFCs allow them.
Common questions
- What is email syntax?
- Email syntax is the set of rules for how an address is written: a local part, one @ and a domain, defined in RFC 5321 and RFC 5322. An address can have perfect syntax and still not exist; syntax only says it is written correctly.
- How do I check if an email address is formatted correctly?
- Paste it, or a whole list, into the checker above and press Check syntax. Each address is tested against the RFC 5321 and 5322 format rules: one @, a local part of at most 64 allowed characters with no doubled or edge dots, and a domain with valid labels and a real-looking top-level domain.
- Does a valid syntax result mean the email address exists?
- No. Syntax checks only how the address is written. An address with perfect syntax can still belong to a mailbox that was closed or never existed. Only the receiving mail server can say whether a mailbox exists, which takes an SMTP mailbox check.
- What characters are allowed in an email address?
- Before the @: letters, digits, dots (not doubled and not at the start or end) and the symbols ! # $ % & ' * + / = ? ^ _ ` { | } ~ -, under RFC 5322. After the @: letters, digits, hyphens and dots, with no label starting or ending in a hyphen. Spaces are not allowed outside a quoted local part.
- Why is a quoted address marked Check instead of Invalid?
- Because RFC 5322 allows it, so calling it invalid would be wrong, but in practice almost every form and sending platform rejects it. On a marketing or sales list a quoted local part is almost always a data error, so it is worth reviewing rather than sending to.
- Is an email format checker the same as an email verifier?
- No. A format checker reads the characters of the address and never contacts anything. An email verifier also looks up the domain's mail servers and asks the server whether the mailbox exists, which is how it finds addresses that would bounce.
- Can I validate email format with a regex?
- A simple regex such as ^[^\s@]+@[^\s@]+\.[^\s@]+$ catches missing @ signs and spaces, which is enough for a signup form. It misses length limits, doubled dots and bad domain labels, which this checker tests one by one; our guide to email validation regex compares the common patterns.
- Does it catch typos like gmial.com?
- Yes, for common misspellings of Gmail, Hotmail, Yahoo, Outlook, iCloud, AOL and Proton, and for endings like .con or .cmo. They are marked Check with the corrected address, because the original is still valid syntax. A typo in a company's own domain can't be spotted from the text alone.
- Is there a limit on how many addresses I can check?
- No row limit. The check runs in your browser, so the only limit is a 25 MB file size to avoid crashing the tab. Lists of 50,000 addresses take a few seconds.
Sources
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? $9 one-time covers 2,500 emails, no subscription
One plan, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.