SSL and TLS Certificate Errors in Email Servers: How They Impact Deliverability

Understanding SSL & TLS in email security

Emails, without encryption, are subject to eavesdropping, modification, or even phishing. SSL SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are cryptographic protocols that protect the transfer of emails while passing through the internet. 

What is SSL/TLS?

SSL and TLS are protocols that secure the message communication between email clients (Outlook, Thunderbird etc) and mail servers. This means sensitive information—like login credentials, financial information and private messages—is safeguarded from being viewed by anyone else without your consent. 

• TLS is the successor of SSL
• TLS 1.2 TLS 1.3 are the current security standards
• Older versions (SSL 3.0, TLS 1.0 and TLS 1.1) are now deprecated 

Because secure email delivery plays a key role in the services provided by email providers and ISPs may require TLS encryption during email exchange.

How does SSL/TLS encryption secure email communication?

• Confidentiality: Ensures that no unauthorized third parties can intercept and read your emails. 
• Authentication: Ensures that the correct mail server is communicating with the correct sender and receiver 
• Data integrity: Prevents the content of email from being modified during transit.
• Compliance: Several regulations (like GDPR, HIPAA, and CCPA) require encryption to protect sensitive email information.

For example, if you email from an account [email protected] through SMTP, your email client will send an SMTP connection to mail. example. com. To avoid interception of the login credentials by hackers or manipulation of the content of the message, this connection, which is further secured with SSL/TLS, is encrypted.

What is the TLS handshake?

When sending an email, the mail transfer agents between the sending and receiving mail servers will execute a TLS handshake to create a secure channel. Here’s what happens:

1. The sending mail server contacts the receiving mail server over SMTP.
2. The receiving mail server offers a TLS certificate.
3. The sending mail server checks if the certificate is valid and trusted.
4. If verification succeeds, an encrypted connection is established, and the email is securely delivered.

Now, when the SSL/TLS handshake fails, the following scenarios can happen:

• If a server doesn’t support encryption, that email will be delivered in plain text and sensitive data can be captured
• If the certificate is out of date or self-signed or untrusted, the email server may refuse to accept the email altogether.
• Some providers will flag non-TLS connections as spam, reducing inbox placement rates.

Spam filters today look for SSL/TLS compliance as an indicator of email trustworthiness. Inadequate encryption from the server can lead to security flags for emails, resulting in poor deliverability. Common consequences of SSL/TLS misconfigurations include: 

• Emails marked as spam. Unencrypted or misconfigured servers increase the risk of landing in spam folders.
• Email rejection (SMTP 550 errors). Some mail servers outright reject emails without proper TLS encryption.
• Insecure connection warnings. Clients like Gmail and Outlook display warnings when an email is sent from a non-TLS server.
• Data exposure risks. Without encryption, sensitive data can be intercepted during transmission.

Common SSL/TLS certificate errors in email servers

1. Expired or invalid certificate

Every certificate has a forced expiration date, and if it is not renewed before that expiration date, then the server will refuse encrypted email connections. When this happens, email clients such as Outlook, Gmail, or Thunderbird give users a warning regarding an expired certificate.

Why it happens:

• The certificate’s expiration date was missed.
• Auto-renewal failed due to misconfigured settings.
• The server is still using an old or revoked certificate.

2. Self-signed certificate errors

Unlike a conventional certificate signed by a trusted Certificate Authority (CA), a self-signed certificate is its own trusted identifier. It can encrypt email traffic, but email clients and servers won’t trust it by default, causing errors.

Why it happens:

• The server is using a self-generated certificate instead of one from a CA.
• The certificate isn’t installed in the trusted certificate store on client devices.

3. Certificate name mismatch

Certificate mismatch errors prevent users from sending and receiving emails. This is because clients and servers refuse to establish secure connections if the hostname doesn’t match. 

Why it happens:

• The SSL/TLS certificate was issued for example.com, but the mail server uses mail.example.com.
• The certificate does not include wildcards (*.example.com) or alternative hostnames.
• The mail client is configured to connect to an IP address instead of a domain name.

4. Certificate not trusted by client

This error means the email client does not recognize the SSL/TLS certificate because it’s issued by an unknown or untrusted CA. When this occurs, clients reject connections, resulting in failed email sending/receiving. 

Why it happens:

• The CA is not recognized or included in the client’s certificate trust store.
• Missing intermediate certificates prevent proper certificate validation.

5. Weak or unsupported encryption protocols

The mail server is using outdated or insecure TLS versions that are no longer supported. Email providers like Google and Microsoft reject connections using outdated TLS versions—many security compliance frameworks require TLS 1.2 or 1.3 for encryption.

Why it happens:

• The server still supports TLS 1.0 or TLS 1.1, which are deprecated.
• The mail client requires TLS 1.2 or higher for security compliance.

6. Incomplete certificate chain

The certificate chain is missing intermediate certificates, making it unverifiable by clients. Email clients reject SSL/TLS connections if they can’t verify the certificate chain. Untrusted certificates trigger security warnings, making users cautious. 

Why it happens:

• The root and intermediate certificates weren’t installed correctly.
• The CA requires a specific chain of trust, but only the end certificate was installed.

7. Revoked or blacklisted certificate

When the SSL/TLS certificate has been revoked by the CA, it becomes invalid. Mail servers may refuse connections from revoked certificates, and mail servers may also refuse connections from revoked certificates.

Why it happens:

• The CA revoked the certificate due to security issues or a compromised private key.
• The domain was flagged for suspicious activity, leading to certificate revocation.

8. Inactive certificate

This error means the certificate is installed but not being used by the mail server. Clients may also still see security warnings even after renewal. 

Why it happens:

• The mail server is still using an old certificate instead of the new one.
• The server wasn’t restarted after installing the new certificate.

9. Outdated security protocol

The server uses weak security settings, exposing emails to attacks. Some email providers reject weak encryption settings, thus resulting in an error.

Why it happens:

• Weak cipher suites (e.g., RC4, 3DES) are still enabled.
• Outdated cryptographic settings make the connection insecure.

10. Outdated encryption algorithm

Insecure hash algorithm used by the certificate such as SHA-1 or MD5 is susceptible to attacks. If the certificate of your email server still uses these obsolete encryption algorithms, email clients and security systems may reject or flag your emails due to security concerns.

Why it happens:

• SSL/TLS certificate was issued years back in SHA-1 or MD5, and has never been updated 

How to troubleshoot and fix SSL/TLS certificate errors

1. Check SSL/TLS certificates on your email server

Before fixing SSL/TLS issues, you need to diagnose the problem. Verifying encryption settings and checking the status of the certificate will help identify misconfigurations.

Tools to verify SSL/TLS certificates

• SSL Labs. Gives you a comprehensive report on the SSL/TLS setup on your server.
• OpenSSL (openssl s_client -connect mail.example.com:465 -showcerts). Command-line tool to inspect certificates.
• MXToolbox. Tests SMTP, IMAP, and POP3 SSL/TLS security settings.

How to check expiration dates and trust chain

• Run the following OpenSSL command to retrieve certificate details:

openssl s_client -connect mail.example.com:465 -servername mail.example.com | openssl x509 -noout -dates

• It will show certificate expiration date, issuer details, and chain of trust. If the certificate is expired, self-signed, or you are missing intermediate certificates, you will need to take corrective action.

2. Fix certificate expiry issues

An expired certificate prevents secure email transmission. Setting up automatic renewal or manually replacing an expired certificate will restore proper functionality.

How to set up auto-renewals with Let’s Encrypt

Let’s Encrypt provides free SSL/TLS certificates with built-in renewal automation using Certbot. To set up automatic renewal:

• Install Certbot on your email server:

sudo apt install certbot

• Request a certificate for your mail server domain:

sudo certbot certonly –standalone -d mail.example.com

• Enable auto-renewal:

sudo certbot renew –dry-run

This ensures your certificate renews automatically before expiration.

Manually renewing and reissuing certificates

In case, you are using a paid SSL certificate from CA (DigiCert, GlobalSign, etc.), follow these steps to manually reissue the certificate: 

• Log into your CA’s portal and generate a new CSR (Certificate Signing Request).
• Upload the new certificate to your email server.
• Restart your mail server (Postfix, Exim, or Microsoft Exchange) to apply the changes.

3. Resolve self-signed certificate problems

Generate a certificate from a trusted CA

To replace a self-signed certificate:

• Create a Certificate Signing Request (CSR):

openssl req -new -newkey rsa:2048 -nodes -keyout mail.key -out mail.csr

• Submit the CSR to a CA (DigiCert, Let’s Encrypt, etc.).
• Install the issued certificate and restart your mail server.

4. Fix hostname mismatch errors

Verify the Common Name (CN) and Subject Alternative Name (SAN)

• Run the following command to check the certificate details:

openssl s_client -connect mail.example.com:465 | openssl x509 -text -noout | grep -E “Subject|DNS”

• Ensure that the CN (Common Name) or SAN (Subject Alternative Name) matches your mail server’s hostname.

Correct mail server configuration

• Update Postfix (/etc/postfix/main.cf):

smtpd_tls_cert_file=/etc/ssl/certs/mail.example.com.crt smtpd_tls_key_file=/etc/ssl/private/mail.example.com.key

• Restart Postfix:

sudo systemctl restart postfix

• For Microsoft Exchange, update the SSL binding using PowerShell:

Enable-ExchangeCertificate -Thumbprint YOUR_CERT_THUMBPRINT -Services SMTP

5. Update encryption protocols and ciphers

TLS 1.0 and TLS 1.1 are outdated and no longer supported by major email providers.

Check supported TLS versions on the email server

• Run this OpenSSL command to check the supported TLS versions:

openssl s_client -connect mail.example.com:465 -tls1_2

• If the connection fails, your server needs TLS 1.2 or 1.3 enabled.

How to enable TLS 1.2 and TLS 1.3 for improved Security

• For Postfix, add the following to /etc/postfix/main.cf: smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

• Restart the service:

sudo systemctl restart postfix

• For Microsoft Exchange, enforce TLS 1.2+ using PowerShell:

Set-SmtpSendConnector -Identity “Send Connector” -TlsAuthLevel MustEncrypt

6. Ensure the correct certificate chain is installed

Check if intermediate certificates are missing

• Run:

openssl s_client -connect mail.example.com:465 -showcerts

• If the intermediate CA certificate is missing, install it on the server.

Steps to update the certificate bundle on your mail server

• Download the correct intermediate certificate from your CA.
• Append it to your existing certificate:

cat mail.crt intermediate.crt > fullchain.crt

• Restart your mail server:

sudo systemctl restart postfix

Preventing future SSL/TLS issues

SSL/TLS certificate errors can disrupt email security if not monitored regularly. Implementing proactive monitoring and automation ensures that your email system stays secure.

• Enable logging on your mail server to detect SSL/TLS errors early.
• Monitor expiration dates and renew certificates before they expire.
• Test email encryption settings using tools like SSL Labs and OpenSSL.
• Keep TLS versions updated (disable SSL 3.0, TLS 1.0, and TLS 1.1).

Automate renewals and certificate validity checks

Use Certbot for auto-renewal

Schedule an automatic certificate renewal with Certbot:

echo “0 0 * * 1 certbot renew –quiet” | sudo tee -a /etc/crontab

This runs every Monday at midnight, ensuring your certificates are always up to date.

Set up alerts for expiring certificates

Use SSL monitoring tools to receive alerts before a certificate expires:

• Let’s Monitor: Sends email notifications for expiring certificates.
• Nagios SSL Certificate Monitor: Automatically checks SSL expiration.

How Warmy helps ensure secure and reliable email deliverability

SSL/TLS certificate errors are just one of the dozens of factors that can affect email deliverability. Even if your email server is correctly configured with valid SSL/TLS certificates, other deliverability issues—like spam filters, poor sender reputation, and DNS misconfigurations—can still prevent your emails from reaching inboxes. This is where Warmy.io comes in.

Strengthening sender reputation to reduce email rejections

Warmy’s AI-powered email warmup ensures that your email domain builds trust with email providers, helping you reduce the likelihood of email rejection. This is done by gradually increasing email volume to prevent email providers from flagging senders as spam. Warmy also helps improve email placement rates—ensuring that secure emails actually reach inboxes.

Comprehensive deliverability insights

Warmy’s free email deliverability test is gold. It checks if your emails are landing in inboxes or being tagged as spam. Warmy also shows the percentage of emails that land in respective folders across major email providers. Warmy’s new Domain Health Hub, for example, takes it one level higher by providing domain-level insights. This means Warmy users can monitor their deliverability at the domain level and access a detailed breakdown of health metrics, performance reports, and deliverability trends.