Fix TLS Unknown_CA Alert in Email Server Connection

What Causes TLS Handshake Failure with Unknown_CA Alert?

You're sending emails, and suddenly the connection drops with a TLS handshake failure. The error log says unknown_ca. You know the server is up, the port is open—but something is blocking it at the trust layer. This isn’t a firewall. It’s a certificate that doesn’t belong.

The TLS handshake fails because the client doesn’t trust the certificate authority (CA) that signed your server’s certificate. This happens when the server uses a self-signed certificate, a private CA, or a CA not in the client’s trust store. Public email services expect certificates from recognized public CAs—like Let’s Encrypt, DigiCert, or Sectigo. When they don’t recognize the CA, the handshake stops dead.

Key takeaways

The unknown_ca alert means the client doesn’t recognize the certificate authority in the server’s chain of trust.
Self-signed certificates and internal/private CAs are common causes because they aren’t in standard trust stores.
Fixing this requires either using a public CA-signed certificate or adding the private CA to the client’s trust store—if you control both ends.

Why Does This Matter for Email Delivery and Verification?

TLS handshake failures with an unknown_ca alert disrupt secure email connections, leading to delivery failures even for valid addresses. This blocks communication between mail servers, causing hard bounces, timeouts, or delayed delivery. When your infrastructure can't complete a handshake, inbox placement tools and verification services can't test or deliver messages reliably, harming sender reputation and long-term deliverability.

How TLS Failures Break the Email Flow

Every email sent between servers relies on TLS to encrypt the connection. If the receiving server’s certificate chain is untrusted—due to an expired, self-signed, or missing CA certificate—the handshake fails with an unknown_ca alert. This doesn’t mean the email address is invalid. It means the connection never completes.

Even if the email address is correct and the content is proper, a failed handshake results in a hard bounce or a timeout. This can be mistaken for a non-existent inbox or a bad domain, especially if you’re not monitoring delivery anomalies closely. Tools like Emaillistchecker.io’s inbox placement test help catch these issues early, but they can’t bypass a broken TLS chain at the infrastructure level.

Why Verification and Deliverability Suffer

If your sender infrastructure has TLS issues, your reputation takes a hit. Major providers like Gmail and Microsoft Outlook track connection reliability. Repeated handshake failures signal poor server maintenance, which leads to higher spam filtering or blocked domains.

Verification tools—including real-time APIs and bulk checks—depend on successful SMTP connections to confirm deliverability. If they can’t establish a secure channel, they’ll flag the recipient as unreachable, even if it isn’t. This inflates your bounce rate and damages reputation metrics.

You can verify hundreds of addresses as valid, but if your own server can’t complete TLS handshakes, your messages won’t land in inboxes. It’s not just about the list; it’s about your entire sending setup. As outlined in RFC 5246, TLS 1.2 and later require a trusted certificate chain—ignoring this breaks end-to-end security.

Check your server configuration regularly. Use tools like MXToolbox or SSL Labs to test your connection chain. Fixing known CA issues prevents bounces and keeps your sender reputation intact. For deeper insight into sender infrastructure health, run an inbox placement test at our inbox placement service—it simulates real-world delivery and flags handshake issues before your campaign goes live.

How to Diagnose the Unknown_CA Alert in Your Setup

When you see a TLS handshake failure with an unknown_ca alert during an email server connection, the issue typically lies in an untrusted or incomplete certificate chain. Use OpenSSL’s s_client to test your server’s handshake and check for verify error:num=20:unable to get local issuer certificate—this confirms the CA signing the certificate isn’t trusted by the client. The root cause is usually a certificate issued by a private or custom CA, or a missing intermediate certificate in the chain.

Step-by-Step Diagnosis Using OpenSSL

Run the command openssl s_client -connect your-mail-server:587 -starttls smtp from your local machine. This connects to your mail server’s SMTP port over TLS and simulates a standard client handshake.
Look for lines in the output containing verify error:num=20:unable to get local issuer certificate or similar. This error means the server’s certificate chain doesn’t lead to a root CA stored in your system’s trust store.
Inspect the certificate chain by checking the output for Server certificate and any issuer lines. If the issuer is something like CN=Internal CA or OU=Company Internal PKI, it’s a private CA. These are never trusted by default in public systems.
If the certificate says it’s issued by Let’s Encrypt, DigiCert, GlobalSign, or another well-known public CA, but still fails, the issue may be a missing intermediate certificate. The chain is incomplete.
Compare the chain against the CA’s official documentation. For example, Let’s Encrypt’s documentation spells out which intermediates are required and how to validate them.

Verify Your Certificate Authority Type

Not all CAs are treated equally. Public CAs (like Let’s Encrypt, DigiCert, Sectigo) are pre-trusted in operating systems and mail clients. Custom or internal CAs are not. If your server uses a custom CA, you must manually install its root certificate into the trust store of every client that connects to it—something rarely feasible at scale.

The 5 steps described in “Step-by-Step Diagnosis Using OpenSSL”, in order.

Even for public CAs, a poorly configured chain—one missing an intermediate—is still untrusted. This is common when certificates are self-exported or misconfigured during setup. Tools like SSL Shopper can help validate chains, but OpenSSL gives you more control and clarity in an actual connection test.

Once confirmed, either reissue your certificate with the correct chain or update your client trust store to include your CA’s root. The fix depends entirely on your setup. If you're using a third-party email service, check their documentation for approved CAs and required chain configurations.

Common Scenarios Where Unknown_CA Occurs

Unknown_CA alerts during TLS handshakes typically happen when your email server can’t verify a certificate because the CA isn’t trusted—commonly due to outdated certificates, private CAs, missing intermediates, or third-party services using untrusted roots. Let’s go over the real-world cases you’ll actually encounter.

Self-Hosted Servers with Let’s Encrypt

You’re using Let’s Encrypt but your certificate bundle is missing intermediate certificates—causing clients to reject the chain, even if your domain is valid.
Your server’s CA bundle is outdated and doesn’t include newer root CAs, especially after Let’s Encrypt phased out the older roots in 2020.
Some automation scripts fail to update the certificate chain properly during renewal—leading to silent TLS failures in outbound email deliveries.
Check your certificate chain using SSL Labs’ SSL Test to see if intermediates are properly included.

Internal Systems and Third-Party Services

You’re running an internal email platform using a private CA (like a company-issued root)—which isn’t recognized by external servers or clients.
Legacy systems or legacy MTAs (like older versions of Exim or Sendmail) don’t include intermediate certificates in the TLS chain, breaking trust.
Third-party APIs or services (especially SaaS tools) present certificates from untrusted CAs—causing handshakes to fail when you connect via SMTP.
Services using unvetted or expired CAs (like some test CA tools) can trigger Unknown_CA in production SMTP exchanges.
If you’re integrating with a new email service, verify its certificate chain matches known standards using IANA’s list of trusted CAs.

These failures aren’t always about your server—you might be hitting a client that doesn’t trust the CA, even if your setup is technically correct. The key is validating the full certificate chain from end to end.

When you’re troubleshooting, start by testing the connection from multiple external points. A tool like inbox placement testing can help reveal where deliverability breaks, including TLS-level issues.

Fix Step-by-Step: Proper Certificate Chain Setup

Fix TLS handshake failures with unknown_ca alerts by ensuring your email server sends the full certificate chain — including intermediates — not just the domain certificate. Skipping intermediates breaks client trust, causing modern mail clients and providers to reject the connection. This step alone resolves 90% of unknown_ca issues in production SMTP setups.

Verify the Certificate Chain

Let’s confirm your server is sending the full chain. Use OpenSSL to test the connection from the outside:

Download the full certificate chain bundle from your Certificate Authority (CA), like Let’s Encrypt. The full chain includes your domain certificate and the intermediate certificate. The CA provider’s documentation (e.g., Let’s Encrypt’s official guide) always shows the correct chain structure for their issuance process.
Ensure your MTA (Postfix, Exim, Sendmail, etc.) references the combined full-chain file — not just your domain cert — in the SSL/TLS configuration. You might call it fullchain.pem or combined.crt. Misconfigurations often point to the raw domain cert alone.
Update your MTA’s configuration file. In Postfix, for example, set smtpd_tls_cert_file = /path/to/fullchain.pem and smtpd_tls_key_file = /path/to/privkey.pem. Ensure the file paths are correct and the file has readable permissions.

Restart the mail service and retest with OpenSSL:

openssl s_client -connect your-mail-server.com:587 -servername your-domain.com

Check the output for a full chain (including intermediates) and no unknown_ca alert.

Common Pitfalls and Checks

Even with the correct chain, issues persist if the server isn’t configured to send it. Let’s Encrypt’s intermediates are standard and widely trusted, but some older or misconfigured MTAs drop them silently. You can validate chain integrity using tools like SSL Labs’ SSL Test — it checks both correctness and completeness of the chain.

Double-check the PEM file content. It should include the domain cert, then one or more intermediate certs, in order. Do not include the root CA certificate — it’s already in clients’ trust stores.

For teams managing email deliverability at scale, ensuring every server presents a valid chain prevents authentication errors, improves inbox placement, and reduces bounce rates. Tools like bulk email verification help catch invalid or suspicious sender setups before they impact sender reputation.

When to Use a Public CA vs. a Custom CA

Use a public CA like Let’s Encrypt or DigiCert for any email server connecting to the public internet. These are trusted by default across all major email clients, web browsers, and mail servers. If you’re running an internal system with no external communication, a custom CA can work—but exposing it externally will trigger unknown_ca alerts and high bounce rates, breaking delivery.

Public CAs: The Default Choice for Public Email Servers

If your email server sends messages to the outside world—via SMTP, API, or web forms—you must use a certificate signed by a public CA. These CAs are pre-trusted by operating systems, email software, and security frameworks. Without this trust, clients refuse the connection during the TLS handshake and return an unknown_ca alert.

Let’s Encrypt, for example, issues free certificates that are trusted by every major platform. The RFC 6844 specification for SMTP over TLS defines how public CAs are handled during transport-layer security negotiation. Public CAs are the only way to avoid handshake failures in the wild.

Custom CAs: Internal Use Only

Private CAs are fine if you control every device and server in a closed network—like a corporate intranet where no external systems connect. But once you try to send to external providers like Gmail, Outlook, or SendGrid, the handshake fails. The receiving server sees a certificate from a CA it doesn’t recognize, and it aborts the connection.

Using a private CA externally leads to consistent unknown_ca failures, which appear in logs as “certificate authority not trusted.” This isn't a configuration issue—it's a fundamental mismatch in certificate trust. Even if the server is otherwise functional, delivery fails at the TLS level.

If you're diagnosing a high bounce rate or inconsistent delivery, check whether you’re accidentally using a private CA in a public-facing role. Most public email services won’t accept mail from servers with self-signed or internal CA certificates. If you're validating your list’s deliverability across providers, use a real public CA. You can test real-world inbox placement with tools like inbox placement testing, which simulates global delivery conditions and reveals TLS handshake issues before you send.

Bottom line: public CAs are non-negotiable for external communication. A custom CA should only live in isolation. Mixing the two breaks TLS trust—and breaks delivery.

Validating Your Fix with Real-World Testing

After adjusting your mail server’s TLS configuration, verify the fix by sending test emails to real domains and checking logs for successful encryption handshakes—no unknown_ca alerts should appear. Use tools that simulate actual delivery to catch hidden issues.

Test End-to-End Delivery with Real Infrastructure

Don’t just verify TLS on your own server—test whether your outbound mail survives real-world scrutiny. Use inbox-placement testing to send a message through your mail server to inboxes at major providers (like Gmail or Outlook) and monitor the full delivery path. This reveals whether your TLS setup holds up under real-world conditions, including strict filtering and certificate checking.

Many mail systems reject messages not because of invalid keys, but due to unrecognized or expired certificate authorities. A failed handshake may look like a connection timeout in logs, but the actual root cause is often a missing CA in your trust store. Testing against real domains confirms your server is trusted by receivers.

Check Logs and Results for Hidden Alerts

Even if delivery appears successful, a subtle unknown_ca alert in the post-transaction log can cause rejection by downstream systems. Run your test again with logging enabled and monitor for any warnings about certificate validation failures.

Tools like OpenSSL’s s_client or vendor-specific probes can show handshake details, but they don’t simulate real inbox placement. The only way to catch this problem early is to send through actual infrastructure and validate both delivery and encryption integrity. This is especially important if you’re using self-signed certs or internal CAs—those often fail with external recipients.

For ongoing validation, consider integrating a real-time verification API into your sending workflow. The email verification API can be paired with delivery testing to ensure both recipient validity and secure connection success. It’s not just about sending mail—it’s about sending it right.

Remember: certificate issues aren’t always apparent in short-term tests. A fix can work today and fail months later if the CA certificate updates or expires. Regular, real-world validation is the only way to maintain reliable delivery.

When your email server hits a TLS handshake failure with an unknown_ca alert, it’s often due to outdated or misconfigured certificates on the receiving end. Emaillistchecker.io catches these issues early by testing not just email syntax, but also the underlying TLS handshake during verification, flagging risky addresses and server configurations before you send to large lists.

Testing Beyond Syntax: Real-Time TLS Validation

Most tools only check if an email address follows the right format. That’s not enough. Let’s be clear: an address can be syntactically valid but still fail delivery due to a broken or misconfigured TLS setup on the recipient's mail server. Emaillistchecker.io’s real-time verification API goes deeper. It connects to each domain’s MX record, attempts a TLS handshake, and flags failures—including unknown_ca alerts—during the process.

This means you see red flags before your campaign launches. If a server’s certificate chain is incomplete or uses an untrusted CA, we report it. This reduces hard bounces and improves sender reputation, especially when sending at scale.

Early Detection Saves Deliverability

Unknown_ca errors are common in large-scale email campaigns—especially when targeting domains with outdated infrastructure. These failures aren’t just about the recipient; they can indirectly hurt your own deliverability. ISPs and inbox providers monitor sender behavior. A high volume of failed TLS handshakes can signal poor hygiene, even if the emails are technically valid.

By identifying these TLS issues before sending, Emaillistchecker.io helps you prune risky addresses and avoid unnecessary failures. It’s not about perfecting your own server config—it’s about understanding which destinations are likely to reject your messages based on their own certificate setup. This kind of proactive filtering prevents wasted sends and helps maintain a clean sender reputation.

For teams using tools like Mailchimp, HubSpot, or SendGrid, integrating Emaillistchecker.io’s real-time API—available at our API page—means you can validate addresses and test TLS connectivity on the fly. You’re not just cleaning a list; you’re hardening your email operations against infrastructure-level delivery risks.

Real-world examples show that even well-maintained email programs encounter unknown_ca errors with certain domains. These can stem from internal corporate policies, outdated compliance certificates, or poor server maintenance. Catching them early gives you a clearer picture of where your messages actually land. You’re not guessing—just verifying. For deeper insight, you can also test inbox placement with our inbox placement tool. It’s one less thing to worry about when you’re focused on getting your message seen.

To learn how this applies to your specific workflow, explore bulk verification or review pricing to get started with your first 100 free verifications.

Best Practices for Maintaining TLS Readiness

Let’s fix TLS handshake failures with unknown_ca alerts by ensuring your certificates are current, chains are complete, and configurations are validated regularly. This means automating renewals, testing chains proactively, and never reusing old certificate bundles — because even one missing intermediate can break mail delivery.

Automate Renewals and Chain Integrity

Use Certbot or another ACME client to automatically renew TLS certificates before expiration — no manual intervention, no downtime.
After renewal, verify the complete certificate chain is served by testing with SSL Labs’ SSL Test or similar tools to ensure intermediates aren’t missing.
Never rely on old certificate files from past setups — outdated or incomplete bundles frequently cause unknown_ca errors even if the root certificate is valid.

Proactively Validate and Audit Certificates

Run automated certificate audits weekly using scripts or monitoring tools (like Prometheus with a TLS exporter) to detect chain issues early.
Test mail server connectivity with real-world receivers — tools like MXToolbox can simulate SMTP handshakes and report TLS handshake failures.
For senders with high-volume outbound mail, validate deliverability using inbox placement testing tools to catch TLS issues before they impact deliverability.

When you send from a server that fails TLS negotiation, email providers often treat it as a signaling risk — even if your content is clean. A single unknown_ca alert can trigger rejection filters and damage sender reputation.

Let’s be clear: a certificate renewal today isn’t enough if the chain isn’t properly configured. The fix is not just in the key or cert, but in the full path from end-entity to root CA. Use tools that check the full chain, not just the server’s own certificate.

For teams managing large email lists, it’s also worth verifying that every recipient domain’s TLS setup is stable. Tools like Emaillistchecker.io’s bulk verification can help isolate and remove invalid or misconfigured email addresses before they become delivery risks.

The Bottom Line on Unknown_CA Alerts in Email Infrastructure

An unknown_ca alert is not a flaw in your email or server configuration — it’s a deliberate security check. The alert means the receiving server cannot verify the certificate chain from your sending server, typically due to a missing or untrusted root in the chain.

Fixing the alert requires valid, publicly trusted certificates with complete, verifiable chains. Workarounds like disabling TLS verification or using self-signed certificates are not safe and increase vulnerability to interception or spoofing.

Proactive validation of your email infrastructure — including certificate trust, DNS records, and sender reputation — helps prevent failures before they impact deliverability. Tools like Emaillistchecker.io offer inbox-placement testing and bulk verification to catch issues like TLS mismatches or invalid domains early.

Sources

DMARC adoption among the world's top 1.8 million domains jumped from 27.2% in 2023 to 47.7% in 2025 — a 75% surge driven by Google and Yahoo's sender rules. — EasyDMARC DMARC Adoption Report 2025 (2025)
By early 2026, 937,931 of 1.8 million analyzed domains had valid DMARC records — up 79% in three years — but about 56% of them still sit at monitoring-only p=none. — DMARC Report (EasyDMARC 2026 data) (2026)

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 'unknown_ca' mean in a TLS handshake?

It means the client does not recognize the certificate authority that signed the server's certificate. The client cannot verify the server's identity, so the connection is blocked for security.

Can a self-signed certificate cause unknown_ca alerts?

Yes. Self-signed certificates are not issued by a trusted CA and will fail verification unless manually added to the client’s trust store.

Do I need to update my email server if I get unknown_ca alerts?

Yes, if the alerts are occurring during outbound mail delivery. The server’s certificate chain must be complete and issued by a public CA.

How often should I test my email server’s TLS setup?

At least monthly, especially after certificate renewals or configuration changes. Automated tools like Emaillistchecker.io help ensure ongoing compliance.

Can bad TLS settings affect my sender reputation?

Indirectly. Frequent handshake failures lead to delivery delays or bounces, which can harm sender reputation over time.

What’s the difference between missing CA and expired CA?

A missing CA means the issuer isn’t in the trust store. An expired CA means the certificate was valid but has passed its expiration date. Both cause TLS failure, but the root causes differ.

Should I disable TLS to avoid unknown_ca errors?

No. Disabling TLS removes encryption and violates best practices. It will also make your server ineligible for most modern email deliverability standards.

How do I test TLS on my email server using free tools?

Use OpenSSL’s s_client command or online services like SSL Labs’ SSL Test. These reveal certificate chain issues, including unknown_ca alerts.

Can a catch-all email address mask a TLS handshake failure?

No. Catch-all setups don’t affect TLS negotiation. If a server cannot handshake, emails will not be delivered regardless of the recipient type.

Does Emaillistchecker.io test TLS during email verification?

Yes. Our real-time API checks SMTP connectivity, MX records, and TLS handshake status to ensure addresses are not only valid but deliverable.

How many free verifications does Emaillistchecker.io offer?

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

Can Emaillistchecker.io help me fix TLS configuration issues?

We detect risks like TLS failures during verification, but we don’t fix server configs. We provide insights so you can address issues in your infrastructure.