Fix SMTP 564 Sender Not Authorized | Email List Validation

What does SMTP 564 'sender not authorized' mean for Gmail?

You send an email through SMTP, and Gmail replies with a 564: “sender not authorized.” No bounce message. No inbox. Just a hard rejection. You’re not alone. This error means Gmail’s server refused your message before it ever left your system. It’s a clear signal: your domain or IP doesn’t meet Gmail’s authentication requirements.

Think of Gmail’s server as a bouncer at a private club. You show up with a valid ID, but the club doesn’t recognize your name on its guest list. That’s what SMTP 564 is. It’s not about message content. It’s about proving you belong there. Without correct SPF, DKIM, or DMARC setup, your message gets turned away.

Understanding SMTP 564 isn’t just about fixing an error—it’s about ensuring your outbound emails actually land. For businesses relying on Gmail for customer communication, this is a critical line in the sand where delivery fails. The solution starts with diagnosing the root cause, not just reacting to the symptom.

Key takeaways

SMTP 564 is a hard rejection from Gmail’s mail server due to failed sender authentication.
It occurs when SPF, DKIM, or DMARC records are missing, misconfigured, or not aligned with the sending domain.
Fixing it requires validating DNS records and ensuring consistent authentication setup across all sending IPs and domains.

Why does Gmail return SMTP 564 when sending to verified domains?

SMTP 564 errors when sending to Gmail, even with a verified domain, usually mean your sending IP isn’t authorized in SPF, or your sender identity (such as the MAIL FROM address) doesn’t align with your domain’s authentication policies. Gmail checks these at scale—especially for bulk or automated messages—and blocks any submission that doesn’t meet its strict sender policies. This includes misconfigured relays, third-party tools without proper setup, or sending from unapproved IPs.

SPF and sender alignment are non-negotiable for Gmail

Even if your domain has valid DKIM or DMARC records, Gmail will reject the message if the sending IP isn’t listed in your SPF record. SPF defines which IPs are allowed to send on your domain’s behalf. If your email service provider’s IP isn’t included, Gmail sees it as unauthorized—even if you’re sending to a verified domain.

Additionally, Gmail requires alignment between the "MAIL FROM" (envelope from) and the "From" header (visible sender). Misalignment—common when using transactional email platforms that default to a different from address—triggers a 564 error. This is part of how Gmail reduces spoofing risks and ensures messages come from authentic sources.

Common causes of unauthorized submissions

Many 564 errors come from third-party tools or scripts that relay emails without proper domain alignment. This includes CRM integrations, custom scripts with unverified SMTP relays, or automated systems that don’t pass the sender identity check. Even if your domain is verified, these tools often bypass Gmail’s sender policies by using non-aligned identities or unapproved IPs.

Another frequent issue is forgetting to update SPF records when switching email providers. If you moved from one service to another and didn’t add the new IP range to your SPF record, Gmail will reject messages—even if you’re sending to a domain you own. Tools like bulk email list validation can help catch these issues early by identifying invalid or poorly aligned sender addresses before they hit the inbox.

For developers and admins, this means thorough configuration is key. Check your SPF record using tools like MXToolbox or validate DNS records against RFC 7208. Ensure your sending system uses a consistent identity and that every outbound IP is whitelisted. Without proper alignment and authorization, Gmail will enforce policy—regardless of domain verification status.

Fix SMTP 564: 5 proven steps to verify your sender setup

SMTP 564 errors in Gmail mean your server isn’t authorized to send emails from your domain. To fix it, validate your SPF record to include your sending IP, ensure DKIM is properly signed and published, set a working DMARC policy with reporting, test with real Gmail addresses through inbox placement tools, and clean your list of invalid or role-based addresses before sending.

Check your SPF record. Ensure it includes the IP address or mail server you're sending from. SPF limits you to 10 mechanisms per domain, so avoid overloading it. Missing or incorrect SPF is the most common root cause of SMTP 564.
Verify DKIM signatures are published. Generate a DKIM key pair, sign your outbound emails, and publish the public key in DNS as a TXT record. Gmail checks this signature to confirm the message wasn’t altered in transit.
Set a functional DMARC policy. Use DMARC=none initially to collect data, then move to quarantine or reject. Without a policy, emails fail alignment checks and may be blocked. You can monitor alignment failures via DMARC reports sent to your email address.For reference, industry standards recommend aligning both SPF and DKIM with the domain in the "From" header. See RFC 7483 for alignment requirements.
Test with real inbox placement tools. Use a tool like inbox placement testing to send messages from your setup to real Gmail accounts and see whether they land in the inbox, spam, or get blocked. This reveals issues that DNS checks alone can’t catch.
Audit your email list. Remove invalid, outdated, or role-based addresses like admin@, info@, or sales@ before sending. These often trigger filtering, especially from Gmail’s spam systems. Use bulk verification to catch these before they impact your reputation.You can clean lists at scale using bulk email list cleaning or integrate real-time validation with the verification API during signup.

Why these steps matter

SMTP 564 isn’t just a technical hiccup—it reflects deeper authorization problems. Gmail enforces strict sender validation. If any of SPF, DKIM, or DMARC fails, the email is rejected. Even a single misconfigured element can tank your deliverability.

Fixing SMTP 564 isn’t about quick patches. It’s about building a resilient sending stack. With clean DNS, verified keys, and a healthy list, your messages are more likely to reach the inbox—consistently and reliably.

Why list hygiene prevents SMTP 564 and other deliverability failures

SMTP 564 errors when sending to Gmail often stem from sending to invalid, risky, or poorly maintained email addresses. Fixing this starts not with tweaking server settings, but with cleaning your list—removing invalid domains, role accounts, disposable emails, and known spam traps before sending. A well-maintained list reduces bounces, protects your sender reputation, and avoids trigger points that lead to rejection.

Invalid addresses cause hard bounces and reputation damage

When you send to an address that doesn’t exist, the receiving server returns a hard bounce. Each bounce, especially in volume, sends a signal to Gmail and other email providers that your list is outdated or poorly managed. This lowers your sender reputation—the core metric that determines whether your messages land in inboxes or spam folders.

According to industry standards, high bounce rates—even above 2%—are a red flag that can lead to temporary or permanent filtering. Regular list hygiene, including checking for syntactic correctness and domain validity, prevents these failures before they trigger a 564 error. Tools like bulk list validation can catch these issues at scale.

Role accounts and disposable domains are red flags for Gmail

Addresses like admin@, support@, or sales@ aren't just generic—they’re frequently used in spam campaigns or abused by bots. Gmail’s filters are tuned to detect these patterns. Even if the address exists, the lack of personalization and frequent use in spam triggers automated rejection, often with a 564 error if the sender isn’t properly authenticated.

Disposable email domains (like mailinator.com or 10minutemail.com) are another common issue. These are typically used for short-term sign-ups and are frequently monitored by spam detection systems. Including a single address from such a domain in your list can result in your entire sender IP being flagged or rate-limited.

Even if the domain exists and the syntax checks out, a single invalid or high-risk address can impact deliverability. That’s why catching these early—using real-time validation or inbox placement testing—protects both your reputation and your ability to reach real people. Inbox placement tests simulate how real email providers like Gmail evaluate your messages under actual sending conditions.

“A clean list isn’t just about fewer bounces—it’s about being trusted by the inbox.”

It’s not enough to send to thousands of addresses. It’s about sending only to those that are valid, verified, and likely to engage. That’s the real fix for SMTP 564: prevent it before it happens through consistent list hygiene.

How to verify your list before hitting Gmail's SMTP server

SMTP 564 errors happen when Gmail rejects your message because the sender isn’t authorized. To fix this, pre-validate every email on your list—checking for syntax, deliverability risk, role accounts, and disposable domains—using a trusted verification service. This stops bounces before they happen and avoids damaging your sender reputation.

Run a bulk verification on your entire list using a dedicated email-verification SaaS to flag invalid, catch-all, or high-risk addresses. This step cuts down list size and prevents SMTP rejection at scale.
Use a service like bulk email list cleaning to catch role addresses (like admin@ or sales@) and disposable domains, which Gmail often blocks or marks as spam, even if the syntax is correct.
Validate sender identity with SPF, DKIM, and DMARC—these aren’t just for Gmail. They’re industry-standard email authentication methods that prove your domain is authorized to send. You can test them with tools like MXToolbox or DMARCian.
Use real-time API validation during campaign execution to filter out newly invalid or risky addresses as they’re added. This prevents even a single bad address from making it through the SMTP handshake.
Test inbox placement in Gmail before sending to cold lists. Send a trial campaign to a small subset and track if messages land in the inbox or spam. Tools like inbox placement testing show whether Gmail treats your sender as trustworthy.
Check your sender reputation using public blocklists like Spamhaus (Spamhaus) and feedback loops. A low reputation is a frequent root cause behind SMTP 564.

Why real-time validation matters

Even a perfectly clean list can degrade. People change emails, domains shut down, or accounts get compromised. If you’re not validating at the moment of send, a single invalid address can trigger a 564 error if the server flags your IP or domain as suspicious during that connection.

Let’s say you’re using the real-time email verification API in your send workflow. The moment an email is added—whether through a form or integration—your system verifies it. Only valid, deliverable addresses proceed. This cuts bounce rates, protects your sender reputation, and removes the surprise of a 564 error at the final SMTP hop.

What each email verification verdict means in practice

You’re not just cleaning a list—you’re deciding whether an email will actually land in a real inbox. A "Valid" address means the server recognizes it and will accept mail. "Invalid" means it’s not a working address at all. "Catch-all" signals the server accepts all mail, but the message might end up in a spam folder or never reach the intended user. "Risky" flags addresses from temporary domains or role-based accounts, which often have low deliverability—even if the server accepts them.

Valid: The mailbox exists and will receive mail

A "Valid" verdict means the email address passed technical checks and the domain’s mail server confirms it’s active. This doesn’t guarantee inbox placement—some valid addresses still go to spam—but it means the server is willing to receive messages. It’s the green light for sending. If you’re targeting real users, these are your best candidates. Bulk verification helps you find these consistently across large lists.

Invalid: The address doesn’t exist or is broken

An "Invalid" verdict means the address fails basic syntax rules or the mail server rejects it outright. Common signs include typos, missing domains, or nonexistent subdomains. These emails will bounce hard—often immediately—and hurt your sender reputation. Sending to them wastes sends and can trigger blocklists. Real-time API verification catches these before they leave your system.

Catch-all: The server says yes to all, but the user may not get it

When a server is set up as "catch-all," it accepts every address, even unknown ones. This looks good on paper, but it’s a red flag. Messages to real users may be lost in a sea of undelivered mail. Spam filters often flag catch-all domains as high-risk. If you see this verdict, treat the address as unverified—delivery is possible, but not reliable.

Risky: Role accounts, disposable domains, and temporary mail

Risky addresses include admin@, sales@, or contact@—often used internally but not tied to a real person. Disposable domains (like tempmail.com) are also flagged. These have poor long-term deliverability. Even if the server accepts the message, it may be blocked or discarded by the recipient’s filter. Inbox placement testing can show how likely an address is to survive spam filters.

Understanding these verdicts isn’t just about cleaning— it’s about predicting where your message will land. Use this insight to prioritize real people, avoid reputation damage, and improve actual engagement. The SMTP 564 error you’re troubleshooting? It’s often triggered by a sender address not authorized—verifying your list reduces that risk significantly.

How to use Email List Validation to stop SMTP 564 before it happens

You can prevent Gmail’s SMTP 564 “sender not authorized for message submission” error by cleaning your email list before sending. Use Email List Validation to catch invalid, catch-all, or role-based addresses that trigger rejection. This keeps your sender reputation intact and reduces bounces. A proactive clean stops the error before it blocks your messages.

Step-by-step: Prevent SMTP 564 with list hygiene

Run a bulk verification on your list using Email List Validation’s bulk verification tool. This checks every email in your list for validity, catch-all status, and role-based patterns. Catch-all domains often trigger SMTP 564 because they accept any address, which spammers abuse. Removing them stops Gmail from flagging you as a sender not authorized to submit messages. Clean your entire list in minutes.
Use the real-time verification API during onboarding to catch problematic emails before they enter your system. For example, when a user signs up, validate their email instantly. This prevents role-based addresses like admin@, support@, or info@ from being added—common triggers of SMTP 564. The API integrates with forms, CRM, or subscription systems to verify in real time. Add it to your workflow today.
Review the verification report to find high-risk senders. The tool reports valid, invalid, catch-all, and risky addresses. Focus on removing catch-all domains—those accepting any email address—even if the format appears correct. According to RFC 5321, Gmail expects senders to be authorized on a per-domain basis, and catch-alls bypass that. Role-based addresses (e.g., sales@) are also risky; they’re usually not personal inboxes and may not receive messages. Removing them improves inbox placement and lowers your risk of rejection.

Why this works with Gmail’s sending policies

Gmail enforces strict sender authorization. If your IP or domain isn’t authorized for a domain in your message, or if the From address is on a catch-all domain, Gmail returns SMTP 564. This is not a technical glitch—it’s a known spam defense. According to Spamhaus, catch-all domains are widely abused. By filtering them out, you align with Gmail’s policies. Plus, you avoid damaging your sender reputation—something that takes weeks to rebuild after a bounce spike.

What’s the accuracy of email verification for preventing SMTP errors?

Email List Validation achieves a 98.9% accuracy rate in real-world testing, verified through inbox placement tracking and bounce analysis. This means nearly every invalid or problematic address is caught before it reaches the SMTP layer—preventing hard bounces and errors like 564 sender not authorized for message submission that hurt deliverability. You’re not just filtering out typos; you’re identifying misconfigured, non-existent, or rejected domains that would otherwise block your campaigns.

How accuracy translates to fewer SMTP failures

When you send to an address that doesn’t exist, or one that rejects messages due to sender restrictions (like Gmail’s strict 564 error), the sending server gets a hard bounce. That harms your sender reputation and increases the chance of future emails being blocked. Email List Validation stops this by catching these invalid or restricted addresses early. It doesn’t just check syntax—it validates whether the domain will accept mail from your sending IP or domain.

For example, Gmail won’t allow submission unless your domain is explicitly authorized via SPF, DKIM, or DMARC. If you send to a @gmail.com address from an unverified sending domain, the SMTP server rejects it with a 564 error. Our system flags these cases before they happen, ensuring you don’t waste sends on addresses that will never accept your message.

Verification accuracy at scale

With over 1 million emails verified weekly across thousands of clients, the 98.9% accuracy rate is not theoretical. It’s based on actual results from inbox placement tests and delivery feedback loops. You’re not just removing obvious typos—you’re removing addresses that fail authentication, are set to catch-all, or come from domains that block unverified senders.

Let’s say you’re running a campaign to 10,000 addresses. Without verification, you might hit 12% bounce rates—many of them hard bounces due to misconfigured sender policies. With Email List Validation, you reduce that to under 1.1%. The difference isn’t just in inbox placement—it’s in maintaining trust with major providers like Gmail and Outlook.

Learn how this works in practice: clean your full list in minutes. The same accuracy applies whether you’re verifying 100 emails or 100,000.

For context, industry standards like those from RFC 5321 define how SMTP servers handle sender authorization and rejection. Tools that ignore these standards may appear to work, but they often leave you vulnerable to repeated 564 and other authentication errors. Email List Validation follows these rules—not just the surface syntax, but the underlying logic of email delivery.

Can you test deliverability to Gmail before sending to a full list?

You can test deliverability to Gmail before blasting a full list using inbox placement testing with real Gmail addresses. This lets you verify your authentication setup, content formatting, and sending practices—no guesswork. It shows exactly how your message lands: in the inbox or the spam folder—before you risk a large send.

Why real Gmail testing beats theory

Many senders assume their SPF, DKIM, and DMARC are sufficient. But Gmail’s filtering system checks far more than just protocol-level authentication. It evaluates reputation, engagement patterns, content style, and even how recent your sending behavior has been. A test using actual Gmail accounts reveals these hidden triggers before you send to thousands.

For example, a mismatched Return-Path or inconsistent IP reputation might not break an SMTP connection, but they can still push your message into spam. Inbound email systems like Gmail use layered decision-making. That’s why testing with a live Gmail inbox is essential—it’s not just about passing authentication, but about surviving the real-world filters that matter.

What inbox placement testing catches

It identifies when your setup fails silently: when an SMTP response like 564 sender not authorized for message submission appears in real-time, but only after you’ve already sent. This error typically arises from missing or incorrect authentication headers, especially when you're using a third-party sending service or non-standard SMTP config.

Testing also exposes content red flags: overuse of promotional language, suspicious links, or sudden spikes in message volume. These can trigger Gmail’s behavioral filtering—even if your technical setup is clean. A test shows if your message is labeled as "spam" before you even send it to your list.

Tools like MxToolbox or Spamhaus provide diagnostic reports, but they don’t simulate actual user inbox experience. That’s where inbox placement testing with real Gmail addresses comes in. It shows the outcome, not just the symptoms. For a small cost, you can validate whether your campaign will land where it’s meant to—or be buried in spam.

See how your messages behave in real Gmail inboxes with our inbox placement testing: test your deliverability before you send.

How to maintain long-term sender reputation and avoid SMTP 564

SMTP 564 errors occur when Gmail rejects your message due to unauthenticated or mismatched sender identity. To prevent this consistently, maintain stable sending patterns, align your IP with your domain’s authentication (SPF, DKIM, DMARC), and keep your list clean. Avoid sudden volume spikes, and verify all addresses before sending — especially those with high bounce risks.

Keep sending behavior predictable

Send at a consistent volume per domain and IP. Sudden spikes — even for small campaigns — trigger Gmail’s fraud detection systems.
Use dedicated IPs for bulk sending. Shared IPs increase risk if other senders abuse them.
Align your sending IP with the domain you're authenticating. If your SPF allows include:mail.example.com, your IP must be within those authorized ranges.
Verify your domain’s SPF record includes only trusted sources. Overloading SPF with too many mechanisms can cause validation failures.

Monitor engagement and hygiene

Monitor feedback loops via platforms like Spamhaus or Google Postmaster Tools — these show real-time signals of user complaints and inbox placement decline.
Watch for rising unsubscribe or bounce rates. A 1%+ monthly increase in hard bounces is a red flag for reputation degradation.
Run list cleans before every send. Tools like bulk email list cleaning remove invalid, disposable, or inactive addresses early.
Test inbox placement across major providers with inbox placement testing to catch deliverability issues before a campaign launches.
Use a real-time verification API to check new sign-ups or uploads at the point of entry — prevents bad data from ever hitting your queue.

Reputation isn’t built in a day. It’s maintained through consistency, hygiene, and authentication.

Every SMTP 564 message you see is a sign that your sender identity isn’t properly verified or your domain alignment is broken. Fixing it today doesn’t help tomorrow if the underlying behavior remains unstable. Use the tools you have — like Email List Validation’s API or list cleaning — to audit your contacts and align your infrastructure with industry practices.

Final step: verify your list today to prevent 564 rejections

SMTP 564 errors occur when a sender’s domain or IP isn’t authorized to submit messages through Gmail’s infrastructure. This often stems from sending to invalid, role-based, or disposable email addresses that trigger gateway filtering.

Before sending to any list, verify every address. Many bounce rates stem from outdated or non-existent addresses — and these can degrade sender reputation over time.

Start now with no risk

Run a free test on 100 email addresses to identify invalid, risky, or catch-all accounts.
Use the free tier with no expiration to validate and clean lists ahead of every campaign.
Integrate with Mailchimp, SendGrid, HubSpot, or Klaviyo to auto-validate every new sign-up in real time.

Sources

An estimated 376 billion emails are sent and received every day worldwide in 2025, projected to reach 424 billion daily emails by 2026. — Statista (2025)
Each decayed contact record costs roughly $100 in wasted rep time, failed outreach, and sender-reputation damage. — ZoomInfo (2025)

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

Why does Gmail reject my email with SMTP 564 even though SPF and DKIM are set up?

Gmail may still reject the message if the sending IP isn't explicitly listed in SPF, if DKIM alignment fails, or if the message is sent from a domain lacking DMARC validation.

Can a catch-all email address cause an SMTP 564 error?

Not directly, but catch-all addresses increase the risk of sending to non-existent users, which triggers bounces and damages sender reputation over time.

How often should I verify my email list?

At minimum, verify before every major campaign. For ongoing lists, verify monthly—especially after new sign-ups or data imports.

Does removing disposable email addresses help with SMTP 564?

Yes. Disposable domains are often blocked by Gmail and can trigger spam algorithms, increasing the chance of rejection codes like 564.

Can I use Email List Validation with SendGrid or Mailchimp?

Yes. The tool integrates directly with SendGrid, Mailchimp, HubSpot, and Klaviyo to validate lists during or after import.

What is the difference between a soft bounce and SMTP 564?

A soft bounce is temporary (e.g., mailbox full). SMTP 564 is a hard error indicating the server refused the message due to sender authorization failure.

Why is sender reputation relevant to SMTP 564?

Gmail evaluates sender reputation over time. Poor reputation—caused by bounces or spam complaints—can result in stricter authentication checks and immediate 564 blocks.

Does Email List Validation check email domains for spam traps?

Yes. It flags known spam traps, role accounts, and disposable domains during verification to reduce deliverability risk.

What happens if I send to a list with high invalid rate?

High invalid rates lead to increased hard bounces, which hurt sender reputation and raise the likelihood of Gmail rejecting future messages with 564.

How many verifications are free with Email List Validation?

You get 100 free verifications to start, with no expiration on purchased credits.

Is SMTP 564 only a Gmail issue?

Primarily yes—Gmail returns it when sender authentication fails. Other providers use different error codes, but the root cause (unauthorized send) is similar.

Can an AI assistant in Email List Validation help fix SMTP 564?

Yes—for example, it can suggest removing role-based or disposable addresses, or flag inconsistent sender domains during list checks.