SMTP 567 Error Code Interpretation

What does SMTP 567 mean and why does it break email delivery?

You're trying to send a time-sensitive update, your app logs show a 567 error, and the email never lands in the inbox. The sender says it’s “failed,” but the log says nothing about content, spam, or invalid addresses.

SMTP 567 isn’t a rejection. It’s a timeout—your server waited too long for a response during the handshake. The recipient’s mail system didn’t acknowledge the connection in time, and your message got dropped before it could be processed.

This error often gets mistaken for 550 (address invalid) or 554 (rejected), but it’s not about the email content. It’s about network delays, configuration mismatches, or the mail server’s inability to respond within the expected window.

Key takeaways

SMTP 567 indicates a failed connection setup due to timeout, not message content rejection.
The error typically results from slow or unresponsive recipient servers, network delays, or misconfigured email infrastructure.
It's not a bounce—no response is received even during the pre-message exchange, making it distinct from 550, 554, or 4xx codes.

How SMTP 567 differs from other common SMTP errors in delivery logs

SMTP 567 means your email session timed out before the server could complete the connection — it’s not about content, policy, or mailbox status. Unlike 550 (recipient not found), 554 (spam/blocked), or 450 (temporary failure), 567 points to infrastructure or network instability during handshake. It’s distinct from 421 (server closing session), which is proactive; 567 is a failure to establish stability during the session itself.

Distinguishing 567 from other SMTP error types

Let’s break down how 567 differs from other common SMTP responses in delivery logs, using real-world behavior.

Error Code	Meaning	Root Cause	Typical Resolution
567	Session timeout	Connection dropped before session finalized — network jitter, server overload, or client-side timeouts	Check network reliability, increase timeout settings, validate SMTP handshake timing
550	Mailbox not found	Recipient address doesn’t exist or is rejected by the recipient’s server	Verify the recipient's email; remove invalid addresses from your list
554	Message blocked	Server blocked due to spam content, sender reputation, or policy (e.g., DMARC rejection)	Review content, sender reputation, and policy alignment — ensure SPF/DKIM/DMARC are set
450	Temporary failure	Server unavailable due to load, maintenance, or rate limiting	Retry with exponential backoff (common in automated systems)
421	Server closing connection	Server intentionally terminated the session — often due to too many requests or policy	Adjust sending rate, avoid spam-like patterns, implement throttling

While 550, 554, and 450 are often about content or policy, 567 is a signal that something went wrong during the TCP handshake or TLS negotiation — a network-level issue. For example, poor routing, firewall interference, or an overloaded mail server can drop the session before the final handshake completes. It’s not about the message, but the conversation.

If you see 567 repeatedly, it’s a red flag that your sending infrastructure or network path is unstable. Bulk verification can help identify whether problematic domains are part of higher failure rates, especially if your list includes outdated or poorly targeted addresses. Tools like inbox placement testing also detect whether your emails reach the inbox despite server-side timeouts.

For more, consult the IETF’s SMTP specification — it clearly defines the 5xx and 4xx classifications and how servers should respond during session negotiation.

Common causes of SMTP 567 errors during email delivery

SMTP 567 errors typically signal a session timeout during the email handshake, most often due to server overload, misconfigured time limits, network instability, aggressive sending policies, or slow DNS resolution. These issues break the connection before the message is sent, especially under high load or poor network conditions. Let’s break down the real-world triggers behind the 567 error.

Server-side load and performance

Receiving mail servers can hit timeout thresholds during peak traffic, especially on shared infrastructure or low-tier hosting. If the receiving server is busy processing other connections, it may not respond in time for your SMTP session.
High-volume email systems like Gmail, Outlook, or Yahoo often throttle or reject connections under heavy load. This is particularly common during business hours or campaign spikes.
Use tools like inbox placement testing to simulate real delivery conditions and catch timeouts before sending to live users.

Configuration and network issues

Too strict idle timeout settings on the receiving server can kill the session prematurely, even if the connection is active. Some configurations time out after 300 seconds — if your session takes longer, the connection drops.
Firewalls or NAT devices between sender and recipient may drop long-lived SMTP sessions, especially on non-SSL ports. This is common in corporate or cloud environments.
Rate limiting from your IP address — due to prior spam activity or high outbound volume — can trigger session drops. Monitor your sender reputation through tools like email verification, which can flag IPs with poor track records.
Slow or inconsistent DNS lookups for MX records delay the TCP connection phase. Even a 5-second delay can push a session beyond the timeout threshold.
Delays in resolving the destination domain’s MX record often stem from misconfigured DNS, high query load, or recursive resolver issues. Check with MXToolbox or RFC 5321 for standard SMTP behavior and expected timelines.

How to diagnose SMTP 567 errors in your email delivery workflow

SMTP 567 errors indicate a session timeout during the email handshake. They typically appear when the connection to the recipient’s mail server takes longer than the configured limit—usually between 60 and 120 seconds. To diagnose them, check delivery logs for timing metadata, trace the full SMTP transaction, simulate sessions using tools like MxToolbox or Telnet, and look for patterns tied to specific domains, geographic regions, or times of day.

Step-by-step diagnosis process

Examine delivery logs for timing metadata. Look for timestamps between connection initiation and the first handshake command. If the gap exceeds 60 seconds, a 567 error is likely a timeout, not a server rejection. Most outbound email engines log this data; platforms like SendGrid or Amazon SES expose connection time in raw delivery reports.
Map the full SMTP transaction path. An error at any stage—HELO/EHLO, MAIL FROM, RCPT TO, or DATA—can stall the session. Identify where the process halts. For example, delays after MAIL FROM often point to sender reputation checks, while stalling after RCPT TO may indicate recipient server policy or throttling.
Simulate the session with Telnet or MxToolbox. Use a command like telnet mail.example.com 587 to manually initiate an SMTP session. Follow the standard flow: HELO, STARTTLS, MAIL FROM, RCPT TO, DATA. If the session hangs at a specific step, you’ve isolated the failure point. MxToolbox’s SMTP tester provides real-time feedback on connection behavior and timeout thresholds.
Correlate errors with domain, region, or time patterns. Check if 567 errors cluster on specific domains—especially those with strict rate limits or greylisting policies—or at peak delivery hours. High-volume sends during business hours may trigger temporary connection timeouts on heavily loaded servers. RFC 5321 defines session timing requirements, but actual limits vary by provider.
Verify your list’s quality before sending. Many 567 errors stem from sending to invalid or poorly maintained addresses. High bounce rates, especially on domains with known catch-all policies, can trigger backend timeouts. Use bulk list verification to remove dead or risky addresses early. Clean your list upfront to improve overall deliverability and reduce connection strain during send.

When to suspect server-side limitations

A 567 error isn’t always your fault. Some servers implement aggressive throttling or reject new connections during high load. This often happens in bulk email environments like corporate inboxes or public providers (e.g. Gmail, Yahoo). If you notice timeouts only with certain domains, the issue may lie at the recipient’s end. In such cases, reducing send volume and adding delays between batches may help avoid triggering their threshold defenses.

SMTP 567 and its impact on sender reputation and deliverability

Repeated SMTP 567 errors—session timeouts during email transmission—signal unreliable sending infrastructure. Email providers track connection stability and may penalize consistent timeouts by lowering your sender reputation, even if your message content is clean. This increases the risk of inbox placement failures and potential flags as a spam source.

How timeouts hurt sender reputation

You might not think about server latency, but consistent 567 errors tell mailbox providers you’re not a stable sender. It’s not just about one failed connection; it’s the pattern. If your infrastructure can’t hold a reliable session with major providers like Gmail or Outlook, that behavior gets logged. Providers use session reliability as a proxy for infrastructure quality and trustworthiness.

Let’s be clear: a single timeout isn’t a problem. But if 5% or more of your messages fail due to connection timeouts, it raises red flags. According to RFC 5321, SMTP sessions should complete within a reasonable timeframe, and repeated timeouts violate expected operational standards. Email services treat this as noise—something often associated with spam or bot activity.

Why deliverability suffers long-term

Even if your emails are high-quality, consistent session timeouts can trigger anti-abuse filters. Providers like Microsoft and Google prioritize sending infrastructure stability when deciding whether to accept, quarantine, or block messages. If your system consistently fails to establish a connection, you’re treated as a higher-risk sender, regardless of content.

High timeout rates don’t just impact one campaign—they degrade your long-term domain and IP reputation. Even if you clean your list, a history of unreliable connections remains in the provider’s evaluation model. This isn’t just a technical hiccup; it’s a deliverability signal. Fixing it requires proactive verification of your sending infrastructure and your email list quality.

Proactive validation helps. Use a tool like bulk verification to weed out invalid, catch-all, or unreachable addresses before sending. Validating your list ensures you’re not wasting resources on addresses that will time out. Combined with monitoring your sending performance, this reduces the chance of repeated 567 errors and protects your reputation over time.

How EmailListChecker.io helps prevent SMTP 567 errors before they happen

You can reduce SMTP 567 errors—caused by session timeouts during connection attempts—by verifying your email list before sending. Invalid, inactive, or disposable addresses often fail to complete SMTP handshakes, triggering timeouts. EmailListChecker.io’s 98.9% accurate real-time verification catches these problematic addresses early, while inbox-placement testing identifies delivery risks before you send.

Bulk verification eliminates unreliable addresses

Run bulk list verification to flag addresses that don’t respond to SMTP checks, reducing the chance of connection timeouts during campaign sends.
Remove addresses with weak deliverability signals—like role accounts (e.g. sales@, info@) or disposable domains—before they cause SMTP session hangs.
Use bulk verification to process thousands of emails at once, cleaning your list before integration with any email service.

Inbox-placement testing catches timeout risks in real conditions

Simulate real-world delivery using inbox-placement testing to spot likely timeout scenarios, including delayed responses or server-side drops.
Check how your message performs across major inboxes (Gmail, Outlook, Yahoo) under actual SMTP conditions—not just DNS or syntax rules.
Find and fix issues like poorly configured SPF, DKIM, or DMARC records that can trigger handshake failures—all before sending a single campaign.
This testing mirrors what happens during a real email delivery, helping you avoid RFC 5321 handshake timeouts that lead to SMTP 567 errors.

Let’s be clear: not every failed connection comes from misconfigured code. Many come from bad data. You don’t need to wait for a 567 error to occur—prevention is faster and cheaper than remediation. With real-time verification powered by a 98.9% accuracy rate, you catch risky addresses early, including those that are inactive, role-based, or hosted on disposable domains.

Integrations with SendGrid, Mailchimp, and HubSpot allow you to send only cleaned, verified messages, cutting down on the number of failed attempts and reducing load on your outbound servers. By using inbox-placement testing, you validate how your email behaves in actual environments—avoiding timeouts caused by server-level filtering.

“SMTP 567 errors are often a sign of list quality issues, not delivery infrastructure problems.”

Stop chasing symptoms. Fix the root cause: your list. With tools that work with your existing workflow, you can maintain sender reputation, avoid blocklists, and improve inbox placement—all without changing your core email provider.

Key SMTP session timeout behaviors to watch for in logs

When your email sessions time out, the error codes alone don’t tell the full story. Instead, look for where the timeout occurs in the SMTP handshake: no response after HELO, missing replies after MAIL FROM, delays during DATA, or consistent timeouts across domains. These patterns signal whether the issue is on your side, the recipient’s server, or a network hiccup. Let’s break down the key signs.

Early handshake timeouts

Connection established, but no response after HELO/EHLO — this usually points to a server-side delay or a firewall silently dropping the initial handshake.
Server responds to HELO but fails to reply after MAIL FROM — common when the recipient server is rate-limiting or has a full outbound queue, especially during peak load.

Data phase and content scanning delays

Timeout occurs during the DATA phase, after RCPT TO — this often reflects the recipient server scanning content (spam, attachments, or macros) before accepting the message. This can take 10–30 seconds or more on busy systems.
Repeating the same SMTP 567 error across multiple domains—especially with the same delay pattern—suggests a sender-side issue like a slow TCP stack, misconfigured DNS, or an underpowered outbound server.

For more context on how SMTP sessions behave under real-world conditions, the SMTP RFC5321 defines expected timing and error responses. While it doesn’t prescribe exact timeouts, it sets the baseline for what’s considered standard behavior during each phase. Real-world delivery systems often exceed those benchmarks due to defensive scanning.

When troubleshooting recurring timeouts, test your list with real-time verification before sending. Emaillistchecker.io’s bulk verification tool helps catch invalid or risky addresses early, reducing the chance of timeouts caused by bad recipients. You can verify your entire list in minutes with a single upload and receive a clean, accurate report.

Practical steps to reduce SMTP 567 errors in outbound email campaigns

SMTP 567 errors often signal a connection timeout during email delivery, usually due to aggressive sending, poor sender reputation, or misconfigured infrastructure. To lower these errors, reduce sending volume per IP, warm up new IPs slowly, validate DNS records, check domain/IP reputation, and clean your list with tools that identify unreliable or time-sensitive email addresses. Let’s walk through each step.

Limit email volume per IP address to prevent rate-limiting triggers on recipient servers. Sending too many messages in a short window prompts defensive responses, including connection timeouts. Most mail servers enforce rate limits based on connections per minute or per hour—exceeding them often leads to SMTP 567 errors. Stick to known send thresholds: start with 100–200 emails per hour per IP, then scale up gradually.
Warm up new IP addresses slowly. New IPs are treated with suspicion. Start by sending to engaged recipients only, using small daily batches over 7–14 days. This builds trust with receiving servers. Tools like bulk verification help pre-screen your list to isolate high-risk or inactive addresses before sending.
Verify your SPF, DKIM, and DMARC records are properly configured. Missing or incorrect records increase the chance of connection delays or outright rejection. Servers check these during handshake—failure can result in timeout-like behavior even if the server is otherwise willing to accept mail. Validate your setup using tools like MXToolbox. A properly authenticated setup reduces suspicion and connection delay risk.
Check your domain and IP reputation using third-party services like Spamhaus or MXToolbox. If your IP or domain is listed on a blocklist, receiving servers may reject or delay connections. Many 567 errors stem from being filtered before the session even begins. Regular checks prevent surprise blocks.
Remove known-bad or timing-sensitive addresses via email verification. Invalid or temporarily non-responsive addresses often cause timeouts during delivery attempts. Use a tool like email verification API to filter out dead, disposable, or catch-all addresses before sending. This reduces the load on your mail server and avoids timeout spikes.

DNS and reputation: foundational steps

Even with perfect sending behavior, poor DNS or bad reputation leads to connection timeouts. SPF and DKIM aren’t optional—they’re how servers verify your authenticity. DMARC gives you enforcement feedback. Without them, you’re sending blind. Double-check your records using RFC standards—specifically RFC 5322 for message format and RFC 7208 for DMARC. These form the basis of modern email trust.

Verification tools matter

Many SMTP 567 errors are preventable. If your list contains addresses that never respond or time out, they’ll drain connection resources. Cleaning your list isn’t just about deliverability—it’s about efficiency. A tool with 98.9% accuracy (like bulk verification) helps isolate problematic addresses before a single message is sent. This reduces server stress and improves your overall sending health.

Why real-time verification is a proven defense against SMTP 567 errors

SMTP 567 errors often mean your mail server timed out trying to connect—common when sending to domains that reject connections abruptly or have strict rate limits. Real-time verification catches these issues before you send by simulating a live connection, identifying domains that time out during the initial handshake. This stops bounces and protects your sender reputation from the harm of sending to unreachable servers.

The problem with static list scrubbing

Many tools just check if an email format is valid or whether a domain exists. That’s not enough. Static checks don’t reflect current server behavior. A domain that worked last week might now time out or block connections due to tightened security policies. Sending to such domains leads directly to SMTP 567 errors, even if the address looks valid on paper.

How real-time checks prevent session timeouts

Real-time verification with a live SMTP session tests the actual behavior of the receiving server. It starts the connection process as your email client would—negotiating the handshake, checking for open ports, and monitoring time-to-response. If a server takes longer than the configured limit (usually 30–60 seconds) to respond, it’s flagged as a timeout-prone domain.

At EmailListChecker.io, our API performs exactly this. It simulates a real send attempt across thousands of domains in seconds, identifying not just invalid addresses but also domains that time out before delivery starts. This includes catch-all setups that appear to accept all emails but actually throttle connections aggressively.

Unlike tools that rely on outdated databases or basic syntax checks, real-time verification reflects what’s happening now. You’re not guessing about server availability—you’re testing it. This means fewer delivery failures, lower bounce rates, and less strain on your mail infrastructure.

The difference is tangible: you send only to domains that are both technically available and receptive. Tools like ZeroBounce or NeverBounce offer similar checks, but EmailListChecker’s 98.9% accuracy and real-time API integration let you embed detection directly into your sending workflow. Verify your entire list before sending and avoid the delays and damage caused by SMTP 567 timeouts.

How to integrate EmailListChecker.io for ongoing deliverability health

You can maintain list hygiene and reduce SMTP 567 errors by validating emails in real time as they're added, running regular bulk checks to purge stale addresses, using the AI assistant to interpret verification results (like catch-all or risky), and syncing clean data directly to Mailchimp, Klaviyo, SendGrid, and HubSpot—all without leaving your workflow.

Integrate the real-time verification API as new contacts enter your CRM or email service. This catches invalid or risky addresses before they reach your outbound queue, reducing the chance of session timeout errors during delivery. According to RFC 5321, SMTP sessions fail early when an address is undeliverable—preventing this early avoids wasted connection attempts. RFC 5321 defines the protocol behavior behind these timeouts.
Schedule automated bulk verification every 30–60 days. Over time, even valid addresses become outdated—on average, 22% of email lists degrade yearly. Regular checks ensure your sender reputation stays strong and inbox placement remains high. Use the bulk verification tool to process thousands at once with precision.
Use the in-app AI assistant to decode results like “catch-all” or “risky.” A catch-all address accepts any email, which inflates your list size without real engagement. A risky tag may signal a temporary or high-bounce domain. The AI helps you decide whether to keep, suppress, or investigate—no guesswork.
Sync verified email lists directly to Mailchimp, Klaviyo, SendGrid, or HubSpot through our native integrations. This ensures your marketing platforms only send to validated addresses, reducing the load on your SMTP servers and lowering the odds of a 567 time-out due to a misrouted session.

Why this prevents SMTP 567 errors

SMTP 567 errors often occur not from the email content, but from a failing connection to an invalid or non-responsive endpoint. When your list contains non-existent or temporarily unavailable addresses, the server times out awaiting a response. By cleaning your list with real-time and bulk checks, you eliminate these dead ends. The result? Fewer timeouts, better sender reputation, and stronger inbox placement.

Keep your system clean and adaptive

Deliverability isn't a one-time fix. It's a process. Let the system do the work. Start with 100 free verifications on the pricing page and see how quickly you reduce bounces. With 98.9% accuracy, EmailListChecker.io helps you act on data, not assumptions.

In summary: fixing SMTP 567 starts with a cleaner, more stable email list

SMTP 567 errors indicate a session timeout during the connection phase — not a content issue. They stem from unreliable or unstable recipient servers, often due to sending to invalid, unresponsive, or high-risk addresses.

Lists with outdated, role-based, or disposable email addresses increase the chance of connection drops and timeouts. These addresses frequently lead to prolonged session negotiations or immediate rejections, disrupting the SMTP handshake.

Proactive list verification removes invalid and risky addresses before sending.
Using EmailListChecker.io ensures you’re only sending to deliverable, responsive inboxes.
With 98.9% accuracy and 100 free verifications that never expire, testing your list carries no risk.

Keep reading

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

Frequently asked questions

Can SMTP 567 errors be caused by a slow internet connection?

Indirectly yes — a slow connection can delay SMTP handshakes, but 567 is more commonly caused by server-side timeouts or network instability between domains.

Does SMTP 567 mean the email address is invalid?

No — it means the server didn’t respond in time. The address might be valid but unreachable, or the server is overloaded.

How long should an SMTP session take before timing out?

Most servers expect response within 60–120 seconds. Timeouts beyond this window trigger error codes like 567 or 421.

Why do some domains consistently return SMTP 567 errors?

These often have aggressive filtering, rate-limiting, or are behind firewalls that drop long-running connections.

Can using a shared IP cause more SMTP 567 errors?

Yes — shared IPs may face higher load or abuse flags, leading to stricter timeouts and higher likelihood of session failure.

How does EmailListChecker.io detect SMTP 567 risk before sending?

It simulates live SMTP connections during verification and flags addresses from domains known to time out or reject sessions.

Are disposable email addresses prone to SMTP 567 errors?

Yes — many disposable domains have short-lived infrastructure that doesn’t maintain stable SMTP sessions, leading to timeouts.

Does DKIM or SPF affect SMTP 567 errors?

No — these verify content authenticity, not connection stability. But poor reputation from missing or misconfigured records can contribute to timeouts.

Is 567 a permanent or temporary error?

It’s a temporary failure. Retrying with rate control may succeed if the issue is transient.

Can I prevent SMTP 567 errors by using a different port?

Port 567 is not standard — SMTP uses ports 25, 465, or 587. Use standard ports and ensure TLS is properly configured.

How often should I verify my email list to avoid 567 issues?

At least monthly, or after significant list growth, to remove stale, invalid, or time-out-prone addresses.

Can a catch-all email address cause SMTP 567 errors?

Yes — catch-all domains often have high server loads, which can lead to timeouts during the SMTP session setup.