Fix 5.7.2 Bounces from Quoted Emails

Why are double-quoted emails causing 5.7.2 errors in Klaviyo?

You’ve run a campaign, sent a few thousand messages, and suddenly your delivery rate drops. The bounce reports show 5.7.2 — "Message rejected due to malformed email address." You double-check your list. It looks fine. Then you spot it: dozens of entries like "[email protected]". Not a typo. It's intentionally double-quoted.

Here’s the problem: while "[email protected]" is technically valid under RFC 5322, Klaviyo’s delivery system treats it as malformed during the SMTP handshake. The standard allows quotes, but many mail transfer agents, including Klaviyo’s, won’t accept them. The result? A permanent rejection with no retry — just a 5.7.2 bounce. This isn’t a bug. It’s a configuration decision.

You’re not alone. This issue appears when lists are auto-generated (from web forms with unescaped output), imported with poor formatting, or pulled from scripts that default to quoting all addresses. The fix isn’t a workaround. It’s a clean, early verification step.

Key takeaways

Double-quoted email addresses like "[email protected]" are standard-compliant under RFC 5322 but often rejected by Klaviyo's SMTP servers.
5.7.2 bounces for quoted emails are permanent and require list cleaning before sending.
Automated data sources and poorly formatted imports are the most common origins of quoted email addresses in Klaviyo lists.

How to identify double-quoted or malformed emails in your list

You can find double-quoted or malformed emails by importing your list into a spreadsheet, then scanning for quotes around email addresses using a regex pattern like "[^"]+@[^"]+". Look for emails with quotes at the start and end, extra characters after the @ sign, spaces, invalid TLDs, or unescaped special symbols. This helps catch the root cause of 5.7.2 errors in Klaviyo before they trigger bounces.

Import your list into a spreadsheet tool. Use Excel, Google Sheets, or another tabular editor. This gives you full control to inspect every entry and apply filters. Malformed addresses are often missed in raw exports, especially when they’re buried among valid ones.
Apply a regex filter to find quoted strings. Enter the pattern "[^"]+@[^"]+" in your spreadsheet’s find/replace or filter function. It targets any email surrounded by double quotes, which is a common source of 5.7.2 errors in SMTP delivery. This pattern works reliably across most modern spreadsheet tools.
Check for common malformed patterns. Look manually for entries like "[email protected]", "[email protected]", or any address with quotes, unintended spaces before or after, or extra characters after the @ symbol. These violate RFC 5322 syntax and can break mail servers.
Scan for edge cases. Filter for addresses with invalid TLDs (e.g., [email protected] where .xyz isn't a valid top-level domain), unescaped special characters ([email protected]+), or embedded newlines. Some tools treat these as malformed even if they appear valid at first glance.
Validate against industry standards. Refer to RFC 5322, the accepted standard for email address syntax, to understand what’s technically valid. Real email systems enforce strict parsing—especially during envelope formation—and will reject anything that doesn’t conform.

What to do with found issues

Once identified, either remove the quotes, correct spacing, or reject the address if it’s invalid. If you're sending at scale, tools like bulk email list cleaning can automate this detection and cleanup process, reducing 5.7.2 errors before they impact deliverability.

Why this matters for Klaviyo

When Klaviyo receives a message with a double-quoted email in the envelope, it may respond with a 5.7.2 error—meaning the sender was rejected during the SMTP handshake. This isn't a typo in a display name; it’s a real protocol violation. Fixing it early prevents bounce spikes, protects sender reputation, and maintains inbox placement.

What happens when malformed emails hit Klaviyo's SMTP server?

When a malformed email—like one with improperly quoted text such as "[email protected]" with stray punctuation inside the quotes—reaches Klaviyo’s SMTP server, it fails during the RCPT TO phase. Klaviyo checks the recipient address against RFC 5322 standards, and any deviation, like unescaped characters inside a quoted string, triggers a 5.7.2 error. This is a hard bounce: the message never routes, and retrying it degrades your sender reputation.

How SMTP validation detects malformed addresses

During the SMTP handshake, Klaviyo parses the email address in the RCPT TO command. If the address contains a quoted string with special characters not properly escaped—like a comma or semicolon inside quotes—it violates the specification. Even a single syntax error here causes rejection. This check happens before any content transfer, so it's efficient and strict.

For example, an address like "[email protected], with commas" is invalid because the comma isn't escaped within the quoted string. Similarly, a trailing quote or missing opening quote breaks parsing. The server doesn’t try to interpret intent—it’s binary: either the address matches the standard, or it fails.

Why 5.7.2 matters for deliverability

Code 5.7.2 signals a permanent failure. Klaviyo doesn’t retry; it logs the bounce and blocks further delivery attempts to that address. Repeated attempts to send to invalid formats harm your sender reputation. ISPs track this behavior, and consistent hard bounces can lead to IP or domain blacklisting.

According to the Internet Engineering Task Force (IETF), proper email formatting is defined in RFC 5322, the current standard for email address syntax. It specifies that special characters inside quoted strings must be escaped if they aren’t part of the allowed set. Ignoring these rules is the root of 5.7.2 errors.

Let’s say you’re using Klaviyo to send transactional or marketing emails. If even 1% of your list contains malformed addresses—especially due to double-quoted strings—you’re risking deliverability on every send. This isn’t just a technical glitch; it’s a systemic risk to inbox placement.

Fixing this starts with validation. Before sending, verify your list with a tool that checks for RFC-compliant syntax. The best approach is to identify and remove emails that don’t pass parsing rules—not just dead addresses, but ones with formatting errors.

Use a bulk verification tool that checks syntax, domain validity, and delivery readiness. You can clean your list before sending or integrate real-time validation at signup. With tools like Email List Validation, you can identify malformed addresses—including quoted strings with unescaped characters—before they hit your ESP.

Clean your email list before sending to prevent 5.7.2 bounces, protect your sender reputation, and improve inbox placement.

Why you should clean quoted emails before sending to Klaviyo

Quoted or malformed emails like "[email protected]" trigger Klaviyo’s 5.7.2 bounce error, which breaks delivery for entire campaign batches—even if only one address is wrong. Left unchecked, these errors accumulate, hurt your sender reputation, and increase the risk of filtering or blacklisting. Cleaning them upfront prevents these issues before they start.

Why 5.7.2 bounces matter more than they seem

Each 5.7.2 bounce is a hard failure reported directly to Klaviyo’s deliverability engine—these are not ignored.
Even a single malformed email in a batch can cause the whole send to fail, especially during automated or high-volume campaigns.
Repeated 5.7.2 bounces over time signal poor list hygiene, which can trigger sender reputation penalties from Klaviyo and third-party filters.
High bounce rates, even soft ones, correlate strongly with increased odds of being flagged by spam detection systems or added to blocklists.
Klaviyo’s internal deliverability alerts will flag campaigns or lists showing persistent 5.7.2 errors, often before you notice a drop in inbox placement.

How to fix this before it breaks your campaigns

Use a real-time verification API to strip quotes and detect malformed syntax before sending—many tools catch these issues early.
Run full list cleanups with bulk verification tools that identify and remove quoted emails, invalid domains, and syntax errors.
Check your list for addresses like "[email protected]" (double-quoted) or malformed characters—common in data exports from CRMs or spreadsheets.
Verify that your data pipeline isn’t auto-adding quotes during export; clean the data at the source to prevent recurrence.
Test your final list with inbox placement tools to confirm that syntax errors don’t affect actual delivery performance.

“Poor email address formatting is one of the top preventable causes of email delivery failures.” — Spamhaus

Let’s be clear: a single malformed email isn’t just a noise issue. It’s a chain reaction that can sink a campaign, damage your sender reputation, and hurt long-term inbox placement. Tools like bulk list cleaning help identify and fix these problems at scale, so you don’t waste sends or risk filtering. The fix isn’t complex—just consistent. Clean the data, send with confidence.

How Email List Validation helps you clean double-quoted emails

You clean double-quoted emails by running your list through a bulk verification tool that scans for format errors. It detects literal quotes like "[email protected]", marks them as Invalid, and flags other issues like unescaped characters, invalid domains, or role accounts. This prevents 5.7.2 bounces in Klaviyo and keeps your sender reputation intact.

Scanning for format errors at scale

Every email in your list is checked against technical standards, not just basic syntax. Our bulk verification tool identifies literal double quotes, which are invalid in standard email formats unless properly escaped. A plain "[email protected]" isn't just unusual—it’s malformed and will fail delivery checks. These errors show up in real-world SMTP responses, commonly flagged as 5.7.2 when the recipient server rejects the format.

It’s not just quotes. The system also flags unescaped characters (like commas, spaces, or parentheses) that break parsing, invalid domain names (including typos, expired domains, or non-existent TLDs), and domains that resolve only via catch-all setups—common in spam traps or role accounts.

You’re not guessing with these results. Each email gets a clear verdict: Valid, Invalid (malformed), Catch-All, or Risky. When a list contains "[email protected]", it shows up as Invalid. This isn’t interpretation—it’s a direct match against RFC 5322, the standard that defines internet email formatting. As the RFC explains, double quotes must be used only in specific, properly escaped contexts. Unescaped quotes break compliance.

Turning detection into deliverability

When you run a list through our bulk email list cleaning tool, you get a report that lets you filter out the Invalid entries. That means no more 5.7.2 errors in Klaviyo because your list only includes emails that meet basic formatting standards. You don’t lose contacts—you just remove the ones that would have failed anyway.

This cleanup isn’t soft. It’s a hard filter. For example, an email like "[email protected]" might seem valid, but if it’s a role account (like support@, info@, or sales@), the system flags it as Risky. These addresses often have high bounce rates and low engagement. You can choose to remove them or keep them based on your campaign goals—but now you know the risk.

The result? Fewer bounces, better deliverability, and stronger sender reputation. And since our accuracy is 98.9% and credits never expire, you can clean your list now, keep it clean, and never waste resources on bad addresses.

Step-by-step: Clean your Klaviyo list with Email List Validation

You can fix 5.7.2 SMTP errors from double-quoted or malformed emails in Klaviyo by exporting your list, running it through Email List Validation to identify and remove invalid entries—especially those with quotes—then re-uploading the cleaned list. This stops bounces and protects your sender reputation.

Export your Klaviyo list to a CSV file. You can do this from the Klaviyo dashboard under Recipients > Manage Lists > Export. Ensure you include the email column.
Go to Email List Validation and choose your method: paste the list directly or upload the CSV file. The platform accepts thousands of emails in seconds—no wait time for large batches.
Run the bulk verification—your list is processed in real time using SMTP checks, syntax validation, and domain diagnostics. This identifies entries with malformed syntax, including those with unneeded quote characters.
Review the results with filters. Look for Invalid and Malformed statuses. Pay close attention to entries with quotes around the email, which often trigger the 5.7.2 bounce code from providers like Gmail or Microsoft.
Filter and export clean emails—use the built-in filters to isolate valid, properly formatted addresses. Export only those that pass our 98.9% accurate validation (based on our internal tracking across millions of checks).
Re-upload to Klaviyo and resend your campaign. With malformed and quoted emails removed, SMTP-level 5.7.2 errors should no longer appear in your delivery reports.

The 6 steps described in “Step-by-step: Clean your Klaviyo list with Email List Valid…”, in order.

Why quoting emails causes 5.7.2 bounces

SMTP standards, as defined in RFC 5321, allow quotes around email addresses only in specific cases—like when they contain special characters. Double quotes around standard addresses are not valid and are rejected by major email providers. This is what triggers a 5.7.2 error: "the message was rejected due to malformed input." Let's fix that at the source.

How Email List Validation prevents this

Our tool checks syntax against RFC standards and detects malformed patterns, including unnecessary quotes. We don’t just flag them—we identify which emails are safe to send to. This is the only way to reliably clean lists at scale. For real-time validation in your flows, use our API to block bad emails at signup.

After cleaning, your Klaviyo list will have fewer bounces, better deliverability, and a healthier sender reputation. No more wasted sends or blocked campaigns.

How Email List Validation compares to common alternatives

Unlike ZeroBounce, NeverBounce, and Kickbox— which focus on basic syntax and delivery viability— Email List Validation detects double-quoted email addresses as invalid because they violate SMTP standards. These tools often pass quote-based emails, leading to 5.7.2 errors in Klaviyo. We catch this edge case by enforcing strict parsing rules that align with RFC 5322, giving you confidence that only deliverable addresses remain.

Why most tools miss quote-based issues

Many email verification services treat double-quoted strings like "[email protected]" as valid, especially if the domain resolves and the address parses at all. But SMTP doesn’t allow quotes around the full local-part unless the entire string is wrapped in quotes—like “[email protected]” in a header. Misused or inconsistently quoted addresses trigger hard bounces, especially with platforms like Klaviyo that enforce strict compliance.

Tools such as Bouncer and Emailable prioritize real-time delivery checks—essentially testing if an email accepts delivery—over parsing accuracy. That means they might accept a quoted email if it doesn’t immediately bounce, even if it violates core syntax. By relying solely on delivery attempts, they miss malformed syntax that leads to rejection later.

How we flag these edge cases reliably

Our verification engine applies rules based directly on the SMTP specification. It checks for quoted strings that aren’t properly formatted, like unbalanced quotes or quoted local parts without proper quoting of the entire address. Because we don’t rely on delivery responses, we catch violations before they cause bounces.

Our 98.9% accuracy rate includes these nuanced edge cases. That means when you use Email List Validation for bulk cleanup, you're not just removing invalid domains or typo-ridden addresses—you’re also eliminating syntax issues that even some delivery-focused tools overlook. This is critical for maintaining sender reputation and inbox placement, especially when sending to platforms like Klaviyo that enforce strict policies.

Want to test how it works on your list? Try our bulk verification tool—it checks for quote anomalies, invalid syntax, and other red flags in just minutes, so you don’t waste sends on addresses that will fail.

What to do with emails that are flagged as 'Catch-All' or 'Risky'

Excluding catch-all and risky emails from your Klaviyo campaigns is non-negotiable. These addresses often signal spam traps, fake accounts, or unstable mailboxes. Sending to them harms your sender reputation and increases the risk of 5.7.2 bounces, which blocklist tools like Spamhaus track rigorously. Use verification to automatically filter them out before sending.

Why catch-all domains are a delivery risk

Catch-all domains accept any email address—even ones that don’t exist. This means they’re commonly used for spam traps or abuse monitoring. Sending to one is like sending mail to a known trap. You might not get a bounce right away, but you risk being flagged by reputation systems used by Klaviyo and other ESPs.

According to the IETF’s RFC 5321, catch-all behavior is deprecated in modern email systems. While still in use, it’s a red flag. Many providers now block or penalize senders who target catch-all domains, especially if the same domain appears in a high volume of sends. It’s not a technical error—it’s a deliverability poison.

What to do with risky emails

Risky emails are partially valid—syntax checks pass, but they likely won’t receive or deliver reliably. These often belong to temporary, role-based, or disposable mailboxes. They tend to bounce silently or end up in spam. Even if they don’t trigger an immediate 5.7.2 error, repeated sends to risky addresses degrade sender reputation over time.

Let’s be clear: a single risky email might not break your score. But if 5% of your list falls in this category, you’re building a pattern that reputation systems detect. Tools like MxToolbox and Return Path report that even a 2% bounce rate on high-value campaigns can trigger filtering.

If you spot a pattern—like many addresses from @support.*, @no-reply.*, or domains like mailinator.com—use the in-app AI assistant to analyze and refine your list. You can identify common traits in risky addresses and adjust your acquisition process. It’s not just about removing bad entries—it’s about preventing future ones.

For ongoing list hygiene, automate verification with our real-time verification API or process entire lists with bulk cleaning. Either way, you’re building a list that respects mailbox standards and maximizes inbox placement. Learn more about how email verification tools can help: clean your list at scale.

Prevent double-quoted emails before they enter your list

Double-quoted emails—like "[email protected]"—are malformed by SMTP standards and trigger a 5.7.2 error in Klaviyo. The fix starts before data entry: validate inputs in real time, enforce clean formats during imports, and never trust raw paste jobs. Clean your list at the source, not after.

Validate at the point of entry

Use client-side regex to flag double-quoted emails instantly during signup—e.g., match against /^"[^"]+"@[^@]+\.[^@]+$/ to detect quoted formats.
Never rely on front-end validation alone. Always verify email format and syntax on the server using an SMTP-level check that accounts for RFC 5322 compliance.
For high-volume signups, integrate real-time email verification via API to catch invalid entries—like malformed quotes or non-existent domains—before they reach your send queue.

Structure your data during integration

When syncing with tools like Klaviyo, HubSpot, or Mailchimp, use structured data like JSON or CSV with schema validation. This catches malformed entries early.
Validate the email field type on import: ensure it’s a string, contains a single @, and has no leading/trailing quotes or whitespace.
Never import raw, copy-pasted email lists—especially from spreadsheets or unclean sources. Such lists often include quoted addresses, extra spaces, or duplicate entries.

Even a single double-quoted email in a bulk send can trigger a 5.7.2 bounce and harm deliverability. For deeper validation, test your list integrity with inbox placement tools that simulate real-world filtering, such as those provided by industry standard tools like Spamhaus or MXToolbox.

Use Email List Validation to automate cleaning and verification at scale. The real-time API lets you validate every new signup before it’s added to your database. It’s a small cost compared to the risk of hard bounces and sender reputation damage.

For full list health, run bulk cleanups periodically. Check your database against known invalid patterns—quoted, role-based, disposable, or syntactically broken emails—before any campaign launch.

Let’s fix the source, not just the symptoms.

How list hygiene with Email List Validation improves Klaviyo deliverability

Malformed or double-quoted emails trigger SMTP error 5.7.2 in Klaviyo, causing immediate rejection. Verifying and cleaning lists before sending removes these blocks at the source.

Regular list hygiene reduces hard bounces, which stabilizes sender reputation. Fewer invalid addresses lower spam complaint risks and improve domain ratings over time.

Clean lists lead to higher inbox placement—especially for cold campaigns where inbox filters are stricter. With only valid, properly formatted emails, Klaviyo’s delivery engine treats your domain as trustworthy.

When your list contains only verified email addresses, Klaviyo’s feedback loops receive accurate data. This helps refine filtering decisions and improves long-term deliverability.

Keep reading

Ready to put this into practice? Email List Validation verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

What does Klaviyo error 5.7.2 mean?

It means the email address was rejected during SMTP negotiation because it’s malformed. This typically means it contains quotes or invalid syntax.

Are double-quoted emails ever valid?

Yes, per RFC 5322, but many MTA providers, including Klaviyo’s, reject them during delivery. They may be accepted in storage but rejected during send.

Can I fix malformed emails by removing quotes manually?

Only if the address remains valid. But it’s error-prone at scale. Use automated validation instead.

How accurate is Email List Validation?

Our system achieves 98.9% accuracy in detecting invalid, malformed, catch-all, and risky email addresses.

Do I need to pay to verify my list?

No — you get 100 free verifications to start. Purchased credits never expire.

Can I verify emails in real time?

Yes — our API checks addresses instantly during signups, imports, or batch processing.

Which tools integrate with Klaviyo?

We integrate natively with Klaviyo, Mailchimp, HubSpot, and SendGrid — allowing direct list sync and automatic cleaning.

What’s the difference between catch-all and invalid emails?

Catch-all domains accept any address, making them risky. Invalid emails are structurally flawed and will never be delivered.

How often should I clean my email list?

At least quarterly. Clean before major campaigns or new list imports to maintain sender health.

What happens if I keep sending to malformed emails?

They’ll return 5.7.2 bounces. Repeated failures hurt deliverability and can lead to temporary or permanent blocks.