SSL and TLS certificates are what let a mail server prove its identity and encrypt the connection during an email exchange. When a certificate is misconfigured, expired, or untrusted, the secure connection can fail entirely — sometimes causing messages to be rejected, sometimes causing them to fall back to an unencrypted (and less trusted) connection instead.
How certificates secure email
When two mail servers exchange a message over an encrypted connection, they perform a TLS handshake: the receiving server presents its certificate, the sending server checks whether it's valid and trusted, and if it passes, an encrypted channel is established. If that handshake fails, the message may be delivered in plain text, rejected outright, or flagged as suspicious by spam filters that look for proper TLS compliance as a trust signal.
Common certificate errors
- Expired certificate: Every certificate has an expiration date. If it isn't renewed in time — often due to a failed auto-renewal — the server will refuse encrypted connections.
- Self-signed certificate: A certificate generated by the server itself rather than issued by a trusted certificate authority. It can still encrypt traffic, but clients won't trust it automatically.
- Hostname mismatch: The certificate was issued for one domain (say, example.com) but the mail server is actually reachable at another (mail.example.com), and the certificate doesn't cover it.
- Untrusted certificate authority: The issuing authority isn't recognized by the connecting client, often because intermediate certificates weren't installed correctly.
- Weak or outdated protocol: The server is still offering deprecated TLS 1.0/1.1 or SSL 3.0, which many modern providers refuse to negotiate with.
- Incomplete certificate chain: Intermediate certificates are missing, so clients can't fully verify the chain of trust back to a root authority.
- Revoked certificate: The issuing authority has invalidated the certificate, often due to a security incident.
How to diagnose the problem
- Use an SSL testing tool to get a full report on your mail server's certificate and supported protocols.
- Check the certificate's expiration date, issuer, and chain of trust directly from the command line if you have server access.
- Confirm the certificate's Common Name or Subject Alternative Name matches the exact hostname your mail server uses.
How to fix common certificate issues
- Expired certificate: Renew it through your certificate authority, or set up automatic renewal if your hosting environment supports it.
- Self-signed certificate: Replace it with one issued by a trusted certificate authority.
- Hostname mismatch: Reissue the certificate for the correct hostname, or request a certificate that covers all the relevant subdomains.
- Weak protocol versions: Disable TLS 1.0, TLS 1.1, and SSL 3.0 on the server, keeping only TLS 1.2 and 1.3 enabled.
- Incomplete chain: Install the full intermediate certificate bundle provided by your certificate authority, not just the end-entity certificate.
Preventing future issues
Enable logging on your mail server so certificate-related failures are easy to spot early, track expiration dates proactively, and periodically re-test your configuration with an SSL testing tool to catch protocol drift before it causes delivery problems.
Common questions
Will a bad certificate stop my emails from sending at all? It depends on the receiving server's policy — some will reject the connection outright, while others will silently fall back to an unencrypted connection, which is a worse outcome from a security standpoint.
How often do I need to renew my mail server's certificate? This depends on your certificate authority, but most modern certificates are valid for 90 days to a year. Automated renewal is strongly recommended so you don't miss the deadline.
Can a certificate issue on my mail server also affect my website? Only if they share the same certificate. It's common (and fine) to use separate certificates for your website and your mail server.
