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Stunnel can put a TLS connection between a legacy mail app and a mail server, or accept TLS connections in front of a mail service. For most users connecting an older local app to a provider, use Stunnel in client mode: the app connects to a loopback address, and Stunnel verifies and encrypts the connection to the provider. The crucial setup choice is whether the remote endpoint expects implicit TLS or STARTTLS; the port and Stunnel protocol handler must match.
What Stunnel does for mail connections
Stunnel is a TLS wrapper for existing services. Its manual describes it as an encryption wrapper between remote clients and local or remote servers, and lists POP, IMAP, and SMTP among supported uses. Stunnel manual
It does not replace mail-server authentication, mail-client account settings, or the server’s access and relay policies. It adds a TLS-protected connection around the service traffic. The two common arrangements are different:
- Client mode: Your local mail app connects to Stunnel over a local plaintext connection. Stunnel makes the TLS connection to the remote mail server and verifies its certificate.
- Server mode: A remote client connects to Stunnel using TLS. Stunnel decrypts that traffic and forwards it to a mail application or service, which still needs appropriate authentication and access controls.
Choose implicit TLS or STARTTLS first
Implicit TLS starts the connection with TLS immediately. STARTTLS begins with the mail protocol in plaintext, then upgrades the connection to TLS after an application-level negotiation. Stunnel’s protocol option supports these STARTTLS handlers for IMAP, POP3, and SMTP; its manual associates them with RFCs 2595, 2449, and 2487. Stunnel manual
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Do not add a STARTTLS handler just because the service is mail. If the destination port expects implicit TLS, connect directly without a STARTTLS handler. If the remote server expects STARTTLS, configure the matching protocol handler and use the provider’s STARTTLS port. Confirm the host, port, and TLS mode with the provider; the examples below are not universal provider settings.
Configure Stunnel in client mode for implicit TLS
The following illustrative IMAP configuration connects a local app to an implicit-TLS endpoint. Replace the hostname, port, and CA bundle path with values confirmed for your provider and operating system.
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[imap]
client = yes
accept = 127.0.0.1:1143
connect = mail.example.com:993
verifyChain = yes
CAfile = /path/to/system-ca-bundle.pem
checkHost = mail.example.com
sni = mail.example.com
sslVersionMin = TLSv1.2In this example, the mail app uses 127.0.0.1 and port 1143 as its incoming server. Stunnel connects to the remote host on port 993, commonly used for IMAP over implicit TLS. The CA file must point to a trusted CA bundle that exists on the system; paths vary by distribution. TLS 1.2 is an example minimum in Stunnel’s HOWTO, not a guarantee that every installed build or local policy supports the same options. Stunnel HOWTO
For client mode, bind accept to loopback unless you deliberately intend other machines to use the local plaintext side. Exposing it beyond the local host could let untrusted systems send traffic through the tunnel.
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Equivalent sample mappings in the official Unix example
Stunnel’s Unix sample illustrates these local-to-remote mappings with certificate-chain and hostname verification. They demonstrate the pattern, not settings guaranteed to work with every provider. Stunnel manual
| Service | Local listener | Remote example | Mode shown |
|---|---|---|---|
| POP3 | 127.0.0.1:110 |
pop.gmail.com:995 |
Implicit TLS |
| IMAP | 127.0.0.1:143 |
imap.gmail.com:993 |
Implicit TLS |
| SMTP | 127.0.0.1:25 |
smtp.gmail.com:465 |
Implicit TLS |
These are documentation examples, not a recommendation to use those ports or hostnames for every account. SMTP providers may offer different submission settings, and POP or IMAP may be disabled or configured differently.
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Configure STARTTLS when the remote server requires it
When the destination requires STARTTLS, use the handler matching the mail protocol in the service section. For example, the protocol option is the application-level negotiation layer; it is not a substitute for choosing the correct STARTTLS destination port. Confirm the exact handler spelling, port, and supported options in the installed Stunnel manual, because available directives can depend on the package build. Stunnel manual Stunnel HOWTO
Keep certificate-chain verification, hostname checking, and SNI appropriate to the destination in the client configuration. Do not copy the implicit-TLS sample unchanged and assume it becomes a STARTTLS setup merely by changing the port: the remote protocol negotiation must match what that service expects.
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Verify the remote server and the installed build
A secure client tunnel should validate both the trust chain and the server identity. verifyChain = yes checks that the certificate chains to a trusted authority, while checkHost checks that it belongs to the expected hostname. Use checkIP for an IP identity where appropriate. Configure SNI for name-based hosting so the remote endpoint can select the intended certificate. Consult the Stunnel HOWTO for option details and the requirements of the installed version. Stunnel HOWTO
Before relying on a directive or TLS version, inspect the build installed on your machine. The HOWTO recommends checking the linked OpenSSL version and available options; stunnel -version and stunnel -options can help identify them. A directive documented for one build may not be available in another. Stunnel HOWTO
Use server mode only when you need a TLS listener
Server mode is for accepting an inbound TLS connection and forwarding decrypted traffic to a backend service. It requires a certificate chain and private key. Restrict access to the listener, protect the private key and its lifecycle, and preserve the backend’s authentication and access rules. Stunnel manual
Take particular care with SMTP forwarding: the official sample warns that an SMTP mapping can create an open relay if the service is not restricted or authenticated. Use network controls and the mail server’s own relay policy rather than assuming TLS alone prevents misuse. Stunnel manual
Quick Recap
Control access and troubleshoot safely
- Connection fails before TLS: Check that the provider hostname and destination port are correct, and that the chosen mode matches the endpoint—implicit TLS or STARTTLS.
- Certificate validation fails: Confirm the CA bundle path, chain trust, hostname or IP check, and SNI value. Do not disable identity verification as a routine workaround.
- A setting is rejected: Check
stunnel -versionandstunnel -options; available features depend on the Stunnel package and linked OpenSSL library. Stunnel HOWTO - A local service is unexpectedly reachable: Bind a client-mode listener to loopback, and use firewall and service-manager controls to limit access to server-mode listeners. Stunnel’s features documentation favors these controls and TLS client authentication over TCP Wrappers for new deployments. Stunnel features
- The mail app still cannot log in or send: Stunnel only wraps the connection. Check the account credentials, mail-client authentication settings, and the backend’s access or relay policy separately.
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