Handling Malformed @ Signs in Emails

Why Extra @ Signs in Email Addresses Break Deliverability

You’re sending to a list. The confirmation emails go out. Then the bounces start piling up—most of them from addresses like user@@example.com. You check the data. It’s not a typo. It’s two @ signs. And that’s not just sloppy—it’s broken by design.

Every email address with multiple @ symbols breaks RFC 5322, the foundation of email formatting. Mail servers don’t wait to see if you mean "user" or "example.com"; they reject the address outright at the SMTP level. This isn’t a deliverability glitch—it’s a protocol violation.

These malformed entries show up in scraped data, poorly curated lists, or form inputs without validation. They don’t just fail—they harm your sender reputation. Each rejected address counts as a failed delivery attempt, which can trigger filtering or blocklist warnings.

That’s where an email verification API handling malformed local parts with extra @ signs comes in: it stops the damage before it starts. You don’t need to scrub lists manually. You don’t need to wonder why your emails aren’t landing.

Key takeaways

Emails with multiple @ symbols violate RFC 5322 and are rejected by mail servers before delivery.
Such addresses commonly appear in scraped or unverified data, causing unnecessary bounces and harming sender reputation.
An email verification API that checks for malformed local parts (like extra @ signs) prevents delivery failures and protects deliverability at scale.

How Malformed Local Parts with Extra @ Signs Are Detected

Our email verification API rejects addresses with multiple @ symbols immediately during syntactic validation, using a strict RFC 5322-compliant regex. This early check stops malformed emails—like user@@example.com—before any DNS or SMTP lookup, saving time, reducing API costs, and preventing network strain.

Early Syntactic Validation Stops Problems Before They Start

You don’t need to send a handshake over the wire to know that an email like "test@@gmail.com" is invalid. The API checks the structure first. If there’s more than one @, it’s automatically flagged as syntactically invalid, based on the standard defined in RFC 5322.

Let’s be clear: no SMTP transaction, no MX lookup, no connection attempt. That’s what makes this step so efficient. It doesn’t wait for network calls to fail—because it never lets them begin.

Why the Early Check Matters in Practice

Every extra @ breaks the email syntax. It’s not a typo or a glitch—it’s a violation of the format that mail systems expect. The API uses a precise regex pattern to catch this, ensuring only well-formed addresses proceed to deeper checks.

When you’re processing thousands of emails, skipping invalid formats early means you avoid wasting API requests on addresses that can never deliver. That lowers latency, reduces costs, and keeps your delivery rates high.

For example, if your list includes user@@company.com, the API rejects it in under 10 milliseconds—before any external service is queried. That’s how real-time verification works at scale. See how it works in action with our email verification API.

What Happens When an API Encounters an Extra @ Sign

If an email address contains more than one @ sign—like user@@example.com—the email verification API immediately flags it as syntactically invalid and returns a specific reason code: syntax-error-multiple-at-signs. This precise verdict distinguishes it from other outcomes like non-existent or catch-all, ensuring you don't misclassify a malformed address as deliverable.

Why Syntax Errors Matter

Email addresses must follow strict formatting rules defined in RFC 5322. An extra @ sign breaks this standard and makes the address unrouteable. Let's say you're processing a list and encounter user@@example.com—without proper syntax validation, this could slip through, inflate your bounce rate, and hurt sender reputation.

Modern email verification APIs, including ours, use this specific error code to separate structural issues from delivery problems. Unlike systems that return generic "invalid" results, our API gives you the exact reason—so you know whether the issue is syntax, domain, or deliverability.

Leveraging This for Real Results

When your bulk verification pipeline encounters syntax-error-multiple-at-signs, it can automatically remove these entries. This means fewer bounces, lower risk of being flagged by ISPs, and better long-term deliverability. We’re not just filtering junk—we’re enforcing standards.

For example, if you're using the email verification API to process thousands of addresses, each malformed entry caught this way prevents a delivery failure from happening. Real-time validation ensures that only valid, properly formatted emails enter your campaigns.

If you're building systems that ingest user data—like signup forms or lead capture tools—you can stop these errors before they become problems. A single extra @ sign won’t cause a mail server to crash, but it will cause a hard bounce, and repeated bounces affect your sender reputation.

DNS and mail routing systems can’t resolve malformed addresses. That’s why the RFC 5322 specification exists: to ensure uniformity. Tools like bulk verification let you scrub entire lists for syntax issues like this before sending. It’s not a small thing—preventing a few bad addresses can improve inbox placement by measurable margins.

The goal isn’t perfection—it’s consistency. An error code like syntax-error-multiple-at-signs gives you clarity, not confusion. And clarity leads to cleaner data, better deliverability, and less wasted effort.

How Emaillistchecker.io's Email Verification API Handles Malformed Local Parts with Extra @ Signs

You don’t need to guess whether an email is valid—our API checks syntax first, using real-time RFC 5322 parsing. Any address with multiple @ symbols is immediately flagged as invalid during the initial syntax pass, with a structured error code so you can filter these cleanly in your automation. No guesswork, no false positives.

Step-by-step: How syntax errors are caught early

Parse using RFC 5322 as the foundation
Every email is checked against the official internet standard for message format. This isn’t a heuristic—it’s a rule-based validation that catches malformed addresses before any further processing.
Scan for multiple @ symbols during syntax pass
Any address with more than one @—like test@@example.com or user@[email protected]—violates standard email syntax. Our API detects this in real time, right after initial parsing.
Mark as 'invalid' with a clear error code
We don’t just return "failed." Instead, we return a structured result with a specific invalid_syntax error code. This enables automated workflows to filter out these cases cleanly—no need to debug vague messages.
Prevent downstream waste
By catching malformed syntax early, we avoid sending verification requests to invalid addresses. This keeps your deliverability score clean and your send volume efficient.

Why this matters for real-world data

Malformed local parts are common—especially in scraped or poorly formatted lists. A single mistyped @ symbol can break an entire campaign. Tools that skip syntax validation or allow these through risk false confidence. RFC 5322, maintained by the IETF, is the definitive reference: https://tools.ietf.org/html/rfc5322.

The 4 steps described in “Step-by-step: How syntax errors are caught early”, in order.

Other services may not flag these errors at the syntax level. Some wait until DNS or SMTP checks, which wastes bandwidth and delays results. We don’t. Our approach ensures only valid, standard-compliant emails proceed to deeper checks.

See how this fits into your workflow: use our real-time API to verify hundreds or thousands of emails instantly with full error transparency.

The Cost of Not Handling Extra @ Signs in Your Email List

Every malformed email address with extra @ signs causes a hard bounce. Left unchecked, these bounces harm your sender reputation, increase spam filter scrutiny, and can push your list into deliverability risk—even with just 5% invalid addresses, your bounce rate may exceed 10%, triggering warnings from major providers.

Hard Bounces Don’t Just Waste Sends — They Hurt Your Inbox Placement

When an email contains two @ symbols, it’s fundamentally invalid by RFC standards. Providers like Gmail and Microsoft reject these immediately with a hard bounce. That’s not just a failed send — it’s a signal to email filtering systems that your sending practices are unreliable. Over time, repeated hard bounces degrade your sender reputation, even if only a small fraction of your list is affected.

Major ISPs monitor aggregate bounce rates. Once you cross the 10% threshold, your messages are flagged for deeper scrutiny. You may end up in spam folders, or worse, blocked entirely. According to industry benchmarks from Return Path (now Validity), lists with sustained bounce rates above 2% start seeing reduced inbox placement, and rates over 5% often result in enforced blacklisting or sender restrictions.

It’s a Small Mistake—But the Chain Reaction Is Real

Think of it this way: one extra @ sign in a user’s input field — maybe from a typo, copy-paste error, or form bug — turns a valid email into a hard bounce. If you’re sending to 10,000 people and 5% are malformed, that’s 500 hard bounces in a single campaign. Even if you clean only part of the list, those bounces still count. There's no "partial" delivery on a malformed address.

Let’s be clear: it’s not just about delivery. It’s about trust. Each hard bounce tells email providers that you don’t validate your data. That erodes the confidence needed to keep your emails in the inbox. And unlike temporary issues, hard bounces are forever in the record.

That’s why verification at scale matters. A real-time email verification API can catch these cases before they hit your sending infrastructure. Tools like EmailListChecker’s verification API validate syntax, check domain existence, and test deliverability—all in real time, without requiring you to run complex DNS queries yourself.

Real-World Example: A Malformed Address in a Bulk List

When you send a bulk email list, malformed addresses like [email protected]@another.company will fail outright—our email verification API catches them instantly during syntax checks. It detects the second @ sign early, categorizes it as a syntax-error-multiple-at-signs, and flags it as invalid before any DNS or SMTP check. This prevents wasted sends, protects sender reputation, and avoids bounces.

How the API Handles Syntax Errors

You input a list with a malformed email: [email protected]@another.company.
The API performs a syntax-level validation first, based on RFC 5322 standards.
It spots two @ symbols—this is not allowed in a valid email address structure.
It returns a clear, machine-readable invalid status with the specific reason: syntax-error-multiple-at-signs.
No resources are wasted on DNS lookups or SMTP handshakes because the address fails at the first gate.

Why This Matters in Production

Malformed addresses like this one often come from poor data entry, flawed imports, or automated scripts. If left unchecked, they trigger hard bounces, hurt deliverability, and can even trigger spam filters.

According to RFC 5322, an email address must contain exactly one @ sign to be syntactically valid. Multiple @ signs violate this core rule, meaning the address is not just suspicious—it’s fundamentally broken.

Let’s say you’re running a campaign with 10,000 contacts. If even 0.1% of them are malformed like this, you're already risking 10 failed deliveries—and that noise can degrade your sender reputation over time.

Using an email verification API is not about filtering out spam. It’s about catching these syntax-level errors before they reach your ESP. Our tool handles it with precision: no false positives, no unnecessary checks.

Start cleaning your list today with our real-time email verification API—it’s the first line of defense against invalid addresses.

How to Handle Malformed Addresses in Your Data Pipeline

You can prevent malformed email addresses like user@@example.com from sabotaging your campaigns by integrating an email verification API early in your data pipeline. Use the API to catch syntax errors like multiple @ signs before they hit your send queue. Flag and log these issues for auditing, so you know where bad data enters your system and can fix it at the source.

Integrate Early, Verify Often

Attach the Emaillistchecker.io API during user onboarding or data sync. Catch syntax errors before data spreads through your systems. This blocks bad inputs at the gate, not after they’ve caused bounces or harm to sender reputation.
Check for the syntax-error-multiple-at-signs response code. This specific code identifies addresses with more than one @ symbol, a clear syntax violation. Most RFC-compliant systems reject such addresses outright, and including them risks triggering spam filters or DNS validation failures.
Log every instance of this error with source metadata. Store the original address, timestamp, and data source (e.g., form submission, CRM import). This creates a traceable audit trail, so you can identify patterns—like a specific third-party feed or outdated signup form—causing repeat issues.

Why It Matters: The Hidden Cost of Malformed Data

Malformed addresses aren’t just rejected—they can harm your sender reputation. Repeated failures with syntax errors may trigger greylisting or temporary blocks on your IP. Some providers treat repeated invalid syntax as signaling low-quality data collection, even if the address is otherwise valid.

The 3 steps described in “Integrate Early, Verify Often”, in order.

According to RFC 5321, the Simple Mail Transfer Protocol explicitly defines the @ symbol as a delimiter, and only one @ is permitted per local-part. Any deviation breaks the standard. Let’s keep your pipeline clean and compliant.

Once identified, you can either correct the data source (e.g., fix a poorly validated form field) or exclude the bad data from your campaign list. The key is catching it early—before it causes bounces, damages deliverability, or inflates your churn metrics.

For full integration, consider using the Emaillistchecker.io API directly in your backend workflow. It handles complex cases like double @ signs, domain mismatches, and catch-all domains, giving you a 98.9% accuracy rate on verification. Use it to sanitize every batch, whether imported from Mailchimp or collected via your web form.

Why You Can’t Rely on Email Clients to Catch This

Email clients like Gmail or Outlook don’t validate full syntax during input—they only check for basic presence of an @ symbol and domain. You can type in an address like user@@example.com and they’ll let you proceed, often silently ignoring the extra @. These malformed addresses only fail when processed by an email service provider during delivery, which checks strict RFC standards. That’s why you need verification at the server level, not just at the user interface.

The Problem Starts With User Input

People make typos. Sometimes they press the @ key twice by accident, or copy-paste data with malformed syntax. Email clients don’t correct this because their job is to help users compose messages, not validate the underlying email format. The system assumes the user knows what they’re doing. But when a message goes through SMTP, that’s where the real validation happens.

Even so, some clients accept addresses with multiple @ signs, not realizing they’re invalid. An address like test@@gmail.com might pass client-side rendering—but it fails SMTP delivery because no RFC-compliant mail server will accept a local part with more than one @.

SMTP Enforcement Is Where It Matters

When an email service provider processes such an address, it checks the full format against RFC 5322, which defines the correct structure of email addresses. Multiple @ symbols are invalid in the local part—those address strings are rejected before any mail transfer occurs.

That’s where systems like the email verification API come in. They catch these issues before you send anything. A full syntax check in your pipeline identifies malformed local parts with extra @ signs—not just at the final delivery stage, but during list preparation.

Let’s say you're syncing leads from a signup form with a typo-rich export. Without pre-delivery validation, those duplicate @ signs get through. You send the email. It bounces. You don’t know why. Your deliverability drops, your sender reputation suffers, and your campaigns stall. The fix isn’t in changing user habits—it’s in validating the input.

With an email verification API, you can catch these errors early. The system uses real-time checks that match the same rules SMTP servers enforce. That includes validating local parts for illegal syntax, including multiple @ signs. You aren’t relying on clients that skip checks. You’re acting on what the actual delivery system demands.

It’s not about being paranoid—it’s about ensuring your message reaches the inbox, not the bounce log. The difference between success and failure often comes down to one malformed character. The right tool can find it before it ever hits a sending server.

How Emaillistchecker.io's Accuracy of 98.9% Includes Syntax Validation

Our 98.9% accuracy isn’t just about whether an email can receive mail—it catches malformed syntax like extra @ signs before any delivery attempt. Every email is validated at the syntax layer first, so invalid formats never make it to DNS or SMTP checks, ensuring only clean, properly structured addresses proceed.

Syntax First: Catching the Basics Early

Let’s be clear: an email like john@@gmail.com isn’t just “risky”—it’s technically invalid. RFC 5322, the standard defining email syntax, explicitly forbids multiple consecutive @ signs. Our API checks this rule first, before anything else. This layer alone filters out common entry errors, typos, and poorly formatted data from your list.

Most tools stop at checking if an email is deliverable. We go further. A valid syntax is the baseline for any reliable verification. Without it, even a perfectly configured SMTP connection can’t help you. If your list includes user@@example.com, it’ll fail at delivery, no matter how good your sender reputation is.

Accuracy Built on Multiple Layers, Not Just Delivery

Our 98.9% accuracy rate reflects performance across four layers: syntax, DNS (MX, SPF), SMTP (connection & response), and inbox placement testing. Malformed entries with extra @ signs are flagged at the very first stage, meaning you don’t waste resources on checks that will fail anyway.

For example, an email like admin@company@com is rejected immediately—it violates basic email formatting rules. We do this before even checking if the domain exists or if the mail server will accept it. This reduces false positives and keeps your deliverability clean.

You can test this yourself with our email verification API, which returns detailed results including invalid_syntax as a verdict. This is transparent and actionable. No guesswork. No wasted sends.

Your Workflow After API Verification: What to Do with Invalid Entries

When your email verification API flags addresses with syntax-error-multiple-at-signs, isolate them immediately. These entries contain invalid syntax—like user@@example.com—and will never deliver. Remove them from your list, or flag them for manual correction. Then export the clean list using the API’s bulk response to ensure your campaigns only go to valid, deliverable addresses. Use the results to update your data collection forms to prevent future input errors.

Process: Clean and Prevent

Identify malformed entries in your verification response. Look for the exact error code syntax-error-multiple-at-signs. This indicates a local part with more than one @ character, which breaks RFC 5322 standards for email format. Even if the domain is valid, these addresses will bounce or be rejected.
Filter and remove all entries with this error from your list. These are not fixable via delivery—it’s a syntax issue at the input level. Removing them reduces bounce rates and protects sender reputation. Use the API's structured response to automate this filtering in your system.
Export the clean list using the bulk verification response. This exports only valid, deliverable addresses—no errors, no risks. You can directly use this file in your email service provider (ESP), or import it into CRM tools. Bulk verification tools like ours make this process fast and reliable at scale.
Update your data collection forms to prevent future input errors. Add real-time validation that blocks input with multiple @ signs. This can be done via JavaScript client-side checks or form validation layers. Preventing bad data at the source is more effective than cleaning it later.
Test your updated form with known invalid patterns (e.g., test@@domain.com) to ensure the validation catches the error before submission. This keeps your database clean and maintainable over time.

Why It Matters: Syntax Errors Are Not Optional

According to RFC 5322, the local part of an email (before @) must not contain more than one @ symbol. Multiple @ signs break parsing, and MTAs (Mail Transfer Agents) will reject such addresses before delivery ever begins. This isn’t a deliverability glitch—it’s a fundamental syntax failure.

Ignoring these errors leads to higher bounce rates, poor sender reputation, and wasted send volume. RFC 5322 defines the standard that every email must follow. Tools like the email verification API detect this early—before you send.

Keep Your List Clean and Your Deliverability Healthy

Extra @ signs in email addresses are a frequent, avoidable error that can break delivery and hurt sender reputation. Even one malformed address in a bulk send can trigger filtering or blocklist alerts.

A robust email verification API with strict syntax validation catches these errors early. Real-time checks prevent invalid data from entering your campaigns before it causes damage.

Emaillistchecker.io uses precise, RFC-compliant checks to detect malformed local parts — including accidental @ signs — before they leave your system. This reduces bounces, maintains trust with inbox providers, and safeguards your domain reputation.

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Ready to put this into practice? Emaillistchecker.io verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

Can an email address have more than one @ symbol?

No. According to RFC 5322, email addresses must have exactly one @ symbol separating the local part and domain.

What does 'syntax-error-multiple-at-signs' mean in email verification?

It means the email address contains more than one @ symbol, making it invalid by email standards.

Does the Emaillistchecker.io API check for malformed local parts?

Yes. The API performs full syntax validation, including checking for multiple @ signs, disallowed characters, and improper formatting.

Why does an address with multiple @ signs fail to deliver?

Mail servers reject such addresses at the SMTP level because they violate the email format standard.

Can a sender reputation be harmed by malformed email addresses?

Yes. Even a small number of invalid addresses with extra @ signs results in hard bounces, which hurt sender reputation.

How often do fake email addresses include multiple @ signs?

They are common in scraped or poorly validated lists, especially those collected from public sources or form submissions.

Can I fix malformed local parts in bulk?

Yes. Once identified via the API, you can filter and correct or remove such entries before sending.

Does Emaillistchecker.io’s free tier include syntax validation?

Yes. The first 100 verifications are free and include full syntax, DNS, and SMTP validation.

How does syntax validation prevent wasted sends?

By rejecting malformed addresses early, it prevents unused API credits and eliminates delivery attempts that would fail.

Is an extra @ sign a sign of spam or fraud?

Not always, but it often indicates poor data quality. It’s a reliable signal of malformed input that should be cleaned.

Can I see the syntax errors in the API response?

Yes. The API returns structured error codes, including 'syntax-error-multiple-at-signs', for each invalid address.

How does Emaillistchecker.io ensure accurate verdicts?

Through layered validation—syntax, DNS, SMTP, and inbox placement—resulting in 98.9% accuracy across all test sets.