What is SMTP 571 and why does it block your emails?

You send a campaign. The email client says “sent.” But the inbox? Empty. No bounce, no delay—just silence. Then you check the logs. There it is: SMTP 571. The server isn’t just rejecting your message. It’s saying, “I won’t touch this at all.”

SMTP 571 is a hard rejection code from the receiving mail server. It’s not a temporary glitch. It means the server refuses to relay your email because of a strict policy—often due to a misconfigured relay, poor sender reputation, or missing authentication.

Unlike soft bounces (like “mailbox full”), 571 errors are final. They don’t disappear with retries. You can’t fix them by sending again. You must address the root cause: the server’s rules, your sending setup, or your IP/domain’s standing in the eyes of the mail recipient.

Key takeaways

  • SMTP 571 is a hard rejection indicating the receiving server refuses to relay your email due to policy restrictions.
  • It commonly occurs when your domain or IP has poor reputation, lacks proper authentication, or is unauthorized to relay mail.
  • Unlike soft bounces, 571 errors are permanent and cannot be retried without fixing the underlying configuration issue.

How does a misconfigured relay policy trigger SMTP 571?

SMTP 571 errors occur when a mail server refuses to relay messages because its relay policy blocks your sending IP or authentication method. If the server only allows mail from authenticated users or approved IPs and your system isn’t on that list, the connection is dropped. This often happens when you try to send through a third-party SMTP service without proper credentials or whitelisting.

Relay policies control who can send via your server

Every mail server uses relay policies to determine which external sources are allowed to send email through it. These rules are designed to prevent abuse, like open relays that spammers exploit. If your server is set to accept mail only from authenticated users or specific IP addresses, any attempt from an unapproved source will be rejected with a 571 error.

Let’s say you’re using a shared SMTP relay hosted by a third-party provider. If their policy demands that all senders authenticate via username/password or are on an approved IP whitelist, and your setup fails either check, the server will return a 571 code. This isn’t a problem with your content — it’s a permission issue.

Authentication and IP approval are key

Even if your message is valid, the server will reject it if your sending IP isn’t in the authorized list. The same applies if your credentials are missing, outdated, or don’t match the required authentication method. Common examples include using an unverified API key, sending from a dynamic IP, or relying on a shared server without explicit allowlisting.

Some providers, like Microsoft 365 and Google Workspace, enforce strict relay policies. If your outbound server isn’t properly verified in their systems, you’ll hit 571 errors consistently. This is why it’s critical to follow the sender policy guidelines for each email service you use.

Before sending bulk mail, verify your sender setup. Tools like bulk email verification can flag problematic addresses and help you pre-validate your list, reducing the risk of hitting relay errors due to sending to invalid or misconfigured destinations.

What’s the difference between a relay policy and a sender reputation issue?

A relay policy controls who’s allowed to send email through your server—like a bouncer checking IDs. Sender reputation is about trust built over time through sending behavior: low spam complaints, low bounces, and consistent volume. A 571 error means your server denied access due to policy, not because you’re untrusted. Even a high-reputation sender gets blocked if the relay policy doesn’t permit them.

Relay policies are server-side gatekeepers

Your mail server uses relay policies to decide whether to accept and forward emails from a given source. If you’re trying to send from an external IP or unapproved domain, and the policy says “no,” you get a 571 error. This is a simple access rule—no nuance, no history. Think of it like a locked gate: if your credentials don’t match the rule, you’re not getting in, regardless of your past behavior.

These policies are often misconfigured in shared hosting environments or when third-party tools try to send via your SMTP server. For example, leaving a “relay allowed from any IP” setting enabled can open your server to abuse. Most serious email providers disable this by default and require explicit, whitelisted access.

Sender reputation is earned behavior

Sender reputation is different. It’s built over time by how other mail servers perceive your sending. High complaint rates, spam traps, or persistent bounces degrade your reputation. Even if your relay policy permits you, a poor reputation can still get your messages marked as spam or blocked entirely.

Major email providers like Google and Microsoft use reputation scores to decide inbox placement. If they see your messages as unwanted, they’ll suppress or redirect them—even if technically delivered. This is why a clean 571 error has nothing to do with your history; it’s purely about permission. You can be trusted with perfect sending habits and still face 571 if the relay policy is locked down.

Tools like inbox placement testing can help you validate how your messages land in real inboxes across major providers. If you’re getting consistent 571 errors, though, the issue isn’t delivery performance—it’s access control.

For deeper insight into how email infrastructure works, you can explore the SMTP specification (RFC 5321)—it details how message delivery is negotiated at the protocol level. Understanding the difference between policy (access) and trust (reputation) is essential when troubleshooting 571 errors or hardening your outbound email flow.

SMTP 571 errors due to misconfigured relay policies usually lack specific details in the response line. If the error message says only “571 Service not available” without a reason like “relay not permitted,” it’s likely a policy issue. Check your mail server’s relay restrictions and review logs to confirm. Reputation problems typically trigger 550 or 554 codes, not 571. Use reputation tools to rule out blacklisting.

Check your full SMTP response for clues

  • Look beyond the 571 code—read the full response line. A plain “571 Service not available” often means policy, not reputation. A specific message like “relay not permitted” points to relay configuration.
  • If the server rejects your connection without a clear reason, it may be blocking you based on a default deny rule. This happens when your IP or domain isn’t explicitly allowed to relay mail through the server.
  • Log the full SMTP transaction—send a test message through inbox placement testing to capture the exact response and see if it changes or remains consistent.

Validate your IP and domain reputation

  • Run your outbound IP through MxToolbox’s blacklist check or Spamhaus to confirm it’s not listed in known spam sources.
  • Reputation issues usually trigger 550 or 554 errors, not 571. If you see a 571 instead, your server may be enforcing a tighter policy while your IP still has a clean reputation.
  • Check your domain’s DMARC, SPF, and DKIM alignment. Misconfigured policies here can cause rejection during authentication—even if your IP is clean, policy blocks override reputation.
  • Review your mail server’s relay settings. If you’re using a third-party relay (like AWS SES or SendGrid), ensure your domain is authorized in their system and relaying is permitted.
Reputation issues don’t usually manifest as 571. If your IP is clean but you still get the error, the root cause is likely a policy or configuration setting, not blacklisting.

Let’s be clear: a 571 without a reason isn’t a spam problem—it’s a permissions problem. If your server denies access with no explanation, the fault is usually in your relay policy, not your sender reputation. Use tools to confirm your IP isn’t blocked, but don’t assume it’s the issue. Focus next on the server’s relay ACLs, TLS settings, and authentication setup.

Step-by-step: Fixing SMTP 571 due to relay policy misconfiguration

SMTP 571 errors occur when your mail server blocks relaying because it doesn’t trust your IP or domain. To fix this, you must ensure your server only allows authorized senders—either through IP whitelisting, explicit domain approval, or verified third-party partnerships. Misconfiguration here stops all outbound mail. Let’s walk through the fixes.

  1. Confirm your SMTP server permits relaying only from approved sources. If your server is set to reject external relays by default, any untrusted IP—like a client or third-party service—will be blocked. Check server logs to see which IPs are being denied, and ensure only those with legitimate access are listed in trusted networks or authentication rules.
  2. Verify your sending domain is in the allowed domains list. Some servers require the domain you're sending from to be explicitly whitelisted. If it’s missing, even if the IP is trusted, the server may reject the message with a 571. Check your server’s domain policy configuration and add any domains used in outbound mail.
  3. Ensure third-party services (SendGrid, Mailgun, etc.) are configured as approved relays. If you're using a service to send email, it must be listed in your server’s relay list or DNS records. For example, Mailgun and SendGrid often require their IP ranges to be whitelisted. Confirm the service is in your server’s approved list or verify it’s authorized in DNS via a designated policy record.
  4. Review server settings to avoid blanket rejections of external relays. If your server is configured to deny all relaying without explicit whitelisting, any unauthorized sender will fail. This includes authenticated users if their domain or IP isn’t in the allowlist. Avoid overly restrictive policies unless necessary—balance security with functionality.
  5. Align SPF, DKIM, and DMARC with your relay setup. If a third-party service sends emails on your behalf, your SPF record must include that service’s IP range. If it doesn’t, the receiving server may flag your emails as spoofed—leading to rejection or spam placement. Use RFC 7208 as a reference for SPF syntax. Similarly, DKIM signatures must be valid, and DMARC policies should align with your sending practices.

Why this matters beyond the 571 error

A misconfigured relay isn’t just a 571 error—it’s a signal to receivers that your email infrastructure is unreliable. Even if you fix the immediate block, inconsistent relay policies can hurt sender reputation. This impacts inbox placement and long-term deliverability, especially with providers that track behavioral signals.

Use tools like bulk email verification to identify invalid or risky addresses before sending, which reduces bounce risk and prevents unnecessary strain on your relay policy. Validating sender domains and addresses regularly helps maintain alignment across your entire outbound flow.

Common relay policy misconfigurations that trigger 571 errors

You get a 571 error when your mail server rejects outbound mail because it’s configured to allow relaying only from authorized sources—but those rules are either missing, too broad, or incorrectly enforced. This usually happens when authentication is required but not properly restricted, when subdomains aren't explicitly allowed, or when SPF policies don't cover your actual sending IPs. Fixing these requires checking the exact logic in your server’s relay settings and aligning them with your sending practices.

Misconfigured authentication and domain restrictions

Forcing authentication on all outbound mail is a good practice—but only if you’re also ensuring that only authenticated senders from authorized domains can relay. If your server allows mail from unauthenticated users on a permitted domain (like example.com), it opens a path for spammers to abuse your relay. A common mistake is enabling relaying for example.com while not restricting access to only known, authenticated internal clients.

Similarly, if your configuration allows only specific subdomains (e.g., sales.example.com, marketing.example.com) to relay, but you send from example.com directly, the server blocks it. This happens more often than you’d expect, especially when teams use tools that impersonate the root domain without including the required subdomain in the relay whitelist.

SPF and catch-all pitfalls

Wildcard SPF records like include:_spf.example.com or v=spf1 -all are commonly misused. If your SPF includes a domain that doesn’t list your actual sending IP or domain as authorized, mail fails SPF checks and may result in 571-style rejections if the receiving server enforces it strictly. You can test this using tools like MXToolbox or RFC 7208.

A catch-all email policy—where every email to an unknown user is accepted—can also trigger 571 errors. This setup allows mail from any domain to be delivered, even if the recipient doesn’t exist, which means your server behaves like an open relay. Modern servers detect this and block delivery. You can verify whether your setup is safe with a real-time inbox placement test before sending to large lists.

How to verify if your list contains addresses that trigger relay policy checks

You can prevent SMTP 571 errors caused by misconfigured relay policies by cleaning your email list before sending. Use an email-verification tool to remove invalid addresses, role-based emails like admin@ or sales@, and disposable domains—these are often flagged by recipient servers as high-risk. A list with high volumes of role addresses triggers automated anti-spam filters, even if your content is legitimate. Testing your list with inbox-placement tools helps you catch delivery failures before they happen.

Filter out high-risk addresses before sending

Role-based addresses, like support@ or info@, are commonly used in spam campaigns, which makes them a red flag for many email providers. When your list contains too many of these, even if they’re valid, they can trigger relay policy rejections. You're not alone—this is a well-documented behavior in industry-wide email authentication practices. The SMTP standard (RFC 5321) explicitly allows servers to reject mail from relays that don’t enforce proper sender policies, which includes unverified or suspicious sender patterns.

Also avoid sending to disposable email domains. Services like temp-mail.org or Mailinator provide short-term inboxes that are frequently used for sign-ups with no real intent to engage. Sending to these increases your chances of being flagged. A tool like bulk verification can process thousands of addresses at once, identifying and filtering out these high-risk entries based on real-time data from over 300 email providers.

Test your list under real delivery conditions

Even a cleaned list can fail if it’s sent to servers with strict policies. That’s why you need to simulate real delivery using inbox-placement testing. These tools send test messages to top inboxes (Gmail, Outlook, Yahoo) and report back whether they land in the inbox, spam, or are blocked. This exposes issues like relay policy checks long before your campaign goes live.

For example, a campaign with a clean list might still fail if the sending infrastructure lacks proper authentication (SPF/DKIM/DMARC) or if the sending domain has a poor reputation. You can check this before sending by using inbox-placement tools that also validate sender reputation and deliverability scores. Test your list in real-world conditions to see if your emails bypass policy-level rejections and reach inboxes consistently.

Why bulk verification helps prevent SMTP 571 by cleaning your sender list

You can prevent SMTP 571 errors caused by misconfigured relay policies by verifying your email list before sending. A clean list removes invalid, disposable, and role-based addresses, reducing outbound volume from unknown sources. This lowers the chance of being flagged by recipient servers that block unauthenticated or high-volume mail from suspicious IPs or domains. Many of these blocks stem from poor sender reputation — which bulk verification helps avoid.

Reducing abuse signals with a cleaner list

When you send to a list full of outdated, malformed, or disposable emails, your outbound volume increases unpredictably. Recipient servers monitor sending patterns and may block traffic if it comes from an IP or domain that doesn’t match known sender behavior. A bulk verification service like bulk verification identifies and removes these problematic addresses before they’re sent, preventing your IP from being flagged as a source of abuse.

For example, role-based addresses like admin@ or sales@ often trigger rejection since they aren’t associated with individuals and can be used for spam. Disposable domains (like mailinator.com) are nearly always rejected. Syntax errors in addresses (like missing @ signs or invalid TLDs) result in immediate bounces. These aren’t just noise — they degrade your sender reputation and increase the likelihood of being blocked by strict relay policies.

How accuracy protects your deliverability

Tools like Emaillistchecker.io use a 98.9% accurate verification process to classify each email as valid, invalid, catch-all, or risky. This precision matters — it’s not just about catching obvious failures, but identifying subtle red flags that affect long-term deliverability. A clean list means your sends are more targeted, more likely to reach inboxes, and less likely to trigger anti-spam systems that enforce relay policies.

When your server isn’t bombarded with invalid deliveries, the behavior of your sending IP appears more consistent and legitimate. This reduces the chance of being marked as a source of unauthorized relaying — the core cause of SMTP 571 errors. The result? Better inbox placement and fewer technical blocks, even when you’re sending at scale.

While no tool can guarantee perfect delivery, a verified list is one of the most consistent ways to stay within acceptable sending behavior. It’s not about avoiding all errors — but about reducing the risk of those that are preventable, including relay policy rejections caused by poor list hygiene.

How Emaillistchecker.io supports fixing SMTP 571 via list hygiene and real-time verification

SMTP 571 errors often stem from sending to domains with strict relay policies, especially those with catch-all addresses or non-existent mailboxes. You fix this not by changing your server, but by ensuring your list only contains valid, deliverable addresses. Emaillistchecker.io prevents these errors by scrubbing invalid domains and catch-alls through bulk verification and catching bad addresses in real time before they ever hit your email service.

Bulk list hygiene stops invalid domains and catch-alls before they trigger 571

  • Run your entire email list through bulk verification to remove domains that reject mail due to relay restrictions or have catch-all policies that block sender verification.
  • Identify and remove catch-all addresses — they’re commonly flagged by strict SMTP servers and often generate 571 errors without sending an actual message.
  • Check domain health and MX record validity to ensure you’re not attempting to send to unresponsive or poorly configured mail servers.
  • Use the bulk verification tool to process thousands of addresses at once and get detailed results on validity, risk, and deliverability.

Real-time verification and integrations prevent bad addresses upstream

  • Integrate the email verification API directly into your signup form or CRM to check every new address as it’s entered — stop invalid data before it enters your system.
  • Automatically reject disposable domains, role-based accounts (like admin@, sales@), and typo-ridden emails that often get flagged by strict relay policies.
  • Sync your verification with top platforms including Mailchimp, SendGrid, HubSpot, and Klaviyo so your lists stay clean across all campaigns.
  • Use real-time API verification to validate individual addresses instantly during onboarding or database import.
  • Verify list quality before deployment with inbox placement testing, which shows how your message will land in real inboxes — a key step before sending to high-risk domains.
Domain-level policies like relay restrictions can break your email flow even if you’re using proper authentication. The fix isn’t changing those rules — it’s not sending to them in the first place.

What to do after fixing the relay policy to avoid future 571 errors

Once you’ve resolved the relay policy misconfiguration, focus on maintaining sender health by tracking bounce types, warming up new IPs and domains slowly, and cleaning your list regularly with verification tools to remove role accounts and disposable inboxes. These steps prevent repeat 571 errors and keep your deliverability consistent.

Track deliverability signals to catch issues early

Even after a fix, your emails can still fail due to changing policies, poor list quality, or declining sender reputation. Monitor bounce types—especially transient (5xx) and permanent (550) errors—and review SMTP response codes in real time. A single 571 error can signal a misconfigured relay, but repeated ones may point to broader reputation or infrastructure issues.

Use tools that log server responses, and set alerts for patterns like spikes in 5xx bounces. This way, you're not waiting for a campaign to fail; you're preventing it before it starts.

Warm up new IPs and domains to build trust

IPs or domains with no sending history are often treated with suspicion. Sending large volumes too soon can trigger automatic blacklisting or rate limiting from providers. Instead, gradually increase your send volume over 7–14 days, starting with low-risk groups like engaged subscribers.

Let’s say you’re sending to 1,000 new subscribers — start with 100, then 200, 300, and so on. This slow ramp helps email providers assess your send behavior and trust your legitimacy. The practice is an industry-standard method backed by providers like Microsoft and Google.

Keep your list clean with real-time verification

Role accounts (e.g., admin@, sales@), invalid emails, and disposable inboxes don’t open messages — they just hurt your sender score. Left unchecked, they can trigger filters that flag your domain as suspicious. Run your list through a tool like bulk verification before every send.

For example, bulk email verification checks domains, syntax, and mailbox existence across real SMTP connections, catching issues like catch-all replies or blocked domains before you send.

Regular cleaning reduces bounces, improves inbox placement, and reduces the risk of triggering a 571 error due to poor list hygiene. It’s not a one-time fix — it’s a habit.

Learn more about how verification works at the underlying level in inbox placement testing, a process that simulates real delivery conditions across major providers.

SMTP 571 is not a delivery failure—it’s a configuration error. Fix it at the source.

SMTP 571 errors signal a policy denial at the receiving gateway, not a flawed message or transient network issue. The root cause is usually an unverified sender, a misconfigured relay, or a list containing invalid or restricted addresses.

Fixing 571 errors starts before sending: validate each email address for authenticity, deliverability, and compliance with relay rules. Tools that report true validity—flagging catch-alls, role accounts, disposable domains, and greylisted addresses—prevent these errors before they happen.

With 98.9% accuracy and credits that never expire, Emaillistchecker.io ensures your list meets the standards mail gateways expect. Clean lists respect relay policies, reduce bounces, and improve inbox placement.

Keep reading

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

Frequently asked questions

What does SMTP 571 mean exactly?

SMTP 571 means the receiving server refuses to relay mail due to a misconfigured policy, often because the sender is not authorized to use that relay server.

Can a good sender reputation cause a 571 error?

No. A good sender reputation helps avoid spam filters but doesn’t override strict relay policies. Even trusted senders get blocked if not explicitly allowed.

Is SMTP 571 a temporary or permanent error?

It’s a permanent error. The server denies the relay attempt entirely. It does not resolve with retries unless the underlying configuration is corrected.

How can I test if my SMTP server will accept my messages?

Use an inbox-placement tool or send test messages from known valid accounts to simulate delivery and catch policy rejections early.

Do catch-all addresses cause SMTP 571 errors?

Not directly. But catch-all servers often enforce stricter relay policies. If your message lands on a catch-all, it may be flagged or blocked even if the address is technically valid.

How often should I verify my email list?

At least monthly for active lists. More frequently if you’re acquiring new addresses rapidly or sending regularly to maintain high deliverability.

Can disposable email domains cause SMTP 571?

They don’t trigger 571 directly, but servers may block them due to high spam ratios. Many disposable domains are rejected at the relay level, which can appear as a 571.

What’s the difference between SMTP 571 and 554?

SMTP 571 is about relay permission; 554 typically indicates the server outright rejects the message due to spam, policy, or blacklisting—often with a specific reason.

How does Emaillistchecker.io help reduce SMTP 571 issues?

By removing invalid, role, and disposable addresses before sending, Emaillistchecker.io reduces the chances of hitting strict relay policies and improves sender integrity.

Do I need to verify every address I send to?

Yes. Even one invalid or poorly configured address can trigger rejection at the relay level. Verifying your full list is the most effective way to avoid SMTP 571.