Configure Postfix as a send-only server by accepting messages from local applications, binding the SMTP listener to loopback, authenticating to an upstream relay over TLS, and retaining an explicit anti-open-relay policy. The host will queue and forward outbound mail, but it will not act as an Internet mailbox or accept arbitrary inbound SMTP connections.
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What “send-only” means in Postfix
A send-only Postfix host performs two separate jobs:
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- Local submission: a web application, monitoring daemon, cron job, or other process hands a message to Postfix on the same machine (or, if deliberately configured, from a tightly controlled application host).
- Outbound delivery: Postfix queues the message and submits it to a named upstream SMTP service, usually over an authenticated and encrypted connection.
It is not a mailbox server. You do not publish it as the MX for a domain, and you do not expose its SMTP listener to the public Internet merely because it needs to send mail. A managed relay is generally the practical choice when the server lacks a stable public IP address or when you do not want to operate DNS, reputation, bounce handling, and direct-delivery policy yourself.
Choose the routing model before editing files
Authenticated relayhost (usually the right choice)
Postfix sends every remote recipient through one configured SMTP endpoint. The relay provider handles the next hop, while your server handles local queueing and submission. This model limits the number of outbound connections you must operate and gives the provider a consistent authentication and reputation boundary.
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Direct delivery to recipient domains
Without a relayhost, Postfix looks up recipient-domain MX records and connects directly. That requires a stable public identity, carefully maintained forward and reverse DNS, suitable sender domains, reputation management, rate control, and the ability to diagnose remote-server policy decisions. A host with a dynamic address or no established sending reputation is more likely to encounter throttling or rejection.
| Decision factor | Authenticated relayhost | Direct delivery |
|---|---|---|
| Network identity | Works when the host has no fixed public sending identity | Requires an address and DNS identity that remote servers will accept |
| Authentication | Uses provider credentials and a SASL password map | Usually relies on the server’s IP and DNS reputation |
| TLS policy | Set the client policy to meet the relay’s submission requirements | Must handle different recipient-server TLS capabilities and policies |
| Reputation and rate limits | Managed partly by the relay service, subject to its limits and terms | Managed entirely by you |
| Operational complexity | Lower: one endpoint, one authentication configuration | Higher: DNS, bounces, throttling, blocklists, and remote SMTP behavior |
Prerequisites and package installation
Install the distribution package
Install Postfix using your operating system’s package manager. Package names and setup dialogs differ by distribution, so select an Internet-site or satellite-system style only when it matches your intended routing. The durable configuration is in Postfix parameters, not in the wording of a particular installer screen.
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Record the installed version before troubleshooting:
postconf mail_versionPostfix releases can differ in defaults and available parameters. Include this version, the distribution release, and the relay provider’s endpoint and port whenever you investigate a failure.
Collect the relay requirements
- The relay hostname and submission port.
- Whether the port expects STARTTLS or implicit TLS (sometimes called wrapper mode).
- The username and password, API-generated SMTP credential, or other SASL identity.
- The sender domains and addresses the account is authorized to use.
- Any provider rate, message-size, or recipient restrictions.
Set the server identity deliberately
Start with a fully qualified host name that the upstream service recognizes:
myhostname = mailout.example.netThen review the related identity parameters rather than accepting values copied from another server:
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| Parameter | Purpose | Configuration question |
|---|---|---|
myhostname |
The server’s canonical name | Does this name match the host identity presented to the relay? |
myorigin |
Domain appended to locally generated addresses without a domain | Should local senders appear under your organization’s domain or the host name? |
mydomain |
The base domain used by related defaults | Is it the domain you intend in envelope and header addresses? |
mydestination |
Domains this machine treats as local delivery destinations | Does it contain only names that should resolve to local mailboxes or local aliases? |
Keep these values consistent with the sender identity permitted by the relay. A message accepted into Postfix’s local queue can still be rejected later if its envelope sender is not authorized upstream.
Bind SMTP to loopback and limit trusted clients
For a host whose applications run locally, keep the SMTP service on loopback:
inet_interfaces = loopback-only
mynetworks_style = hostinet_interfaces = loopback-only prevents Postfix from listening on public interfaces. mynetworks_style = host tells Postfix to trust only the local machine. The broader subnet modes can trust more systems than intended, which is unsafe on a wide-area network.
If a separate application host must submit mail, do not open Postfix to every address. Define a precise network list instead:
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Review IPv4 and IPv6 behavior. A loopback-only policy should cover the loopback addresses your system actually uses. If IPv6 is disabled or intentionally excluded, set the Postfix protocol family accordingly rather than leaving an unintended listener.
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Point Postfix at the upstream relay
Set relayhost to the provider’s submission endpoint. Square brackets suppress MX lookups, forcing Postfix to connect to that exact host:
relayhost = [mail.example.com]:587Use the port and TLS mode documented by the provider. Do not copy an implicit-TLS example to a STARTTLS service or vice versa.
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STARTTLS on a submission port
For a service that starts in plain SMTP and upgrades the connection with STARTTLS, use its documented submission port and require encryption:
relayhost = [mail.example.com]:587
smtp_tls_security_level = encryptWith this policy, Postfix must negotiate TLS before sending the message. The exact port is provider-specific; 587 is a common submission choice but is not a universal requirement.
Implicit TLS (wrapper mode)
For a service that expects TLS immediately when the TCP connection opens, configure the endpoint and wrapper mode together:
relayhost = [mail.example.com]:submissions
smtp_tls_security_level = encrypt
smtp_tls_wrappermode = yesUse wrapper mode only when the provider explicitly requires implicit TLS. A wrapper-mode setting on a STARTTLS port generally causes a protocol mismatch.
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TLS protects the SMTP session and the SASL authentication data while they travel between Postfix and the relay. It does not by itself guarantee that every later hop to a recipient domain remains encrypted; your relay provider controls that onward delivery.
Enable SMTP-client SASL authentication
Tell the Postfix SMTP client to authenticate and identify a protected password map:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymousThe map backend must exist on your distribution. The example uses a hash map; another backend may be installed instead. The map key must match the relay destination as Postfix sees it, including the port when that is part of the configured endpoint. A typical source file is:
[mail.example.com]:587 [email protected]:REDACTED_SECRETUse the exact syntax required by your chosen map type and relay. Never put a real credential in a tutorial, ticket, shell history, or diagnostic log.
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- Create the source file as root and restrict it to the Postfix administrator, for example with mode
600. - Run the map-generation command for the selected backend, such as
postmap hash:/etc/postfix/sasl_passwdwhen using the hash example. - Verify that the generated database file has permissions that prevent unprivileged users from reading it.
- After changing credentials, rebuild the map before reloading Postfix.
Some distributions separate the Postfix SMTP client and SASL implementation into packages. If authentication is enabled but no mechanism is available, install the distribution’s client-side SASL module rather than weakening the relay policy.
Keep the anti-open-relay guard
Send-only does not mean “permit every relay request.” Postfix still needs an explicit recipient authorization policy:
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smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destinationpermit_mynetworks authorizes the tightly scoped trusted networks, and permit_sasl_authenticated authorizes authenticated submitters if you allow them. reject_unauth_destination is the critical guard: it rejects attempts to relay to destinations for which the server is not an authorized final destination unless an earlier permit applies.
Postfix 2.10 and later use smtpd_relay_restrictions for this policy. Older configurations may place the equivalent guard in smtpd_recipient_restrictions. Do not replace the rejection with a blanket permit, and do not assume that binding to loopback makes a permissive policy safe: a compromised local application could still abuse it.
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A compact starting configuration
Adapt the following values to your operating system, relay, identity, and TLS mode:
myhostname = mailout.example.net
myorigin = $mydomain
mydomain = example.net
inet_interfaces = loopback-only
mynetworks_style = host
relayhost = [mail.example.com]:587
smtp_tls_security_level = encrypt
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destinationFor implicit TLS, change the relay endpoint and add smtp_tls_wrappermode = yes as shown earlier. Keep the wrapper setting out of a STARTTLS configuration.
Apply changes and verify effective settings
- Check the values Postfix will actually use, not only the lines you edited:
postconf myhostname relayhost inet_interfaces mynetworks smtp_tls_security_level - Build or update the SASL password map.
- Run the configuration validator:
postfix check - Reload the service without interrupting queued mail:
postfix reload - Submit one controlled message from the local application or a local command.
- Follow the mail log and correlate the queue ID with the relay host, TLS negotiation, authentication result, relay response, and final delivery status.
Service-manager commands and log paths vary by distribution. Common systems write to a mail log under /var/log or to the system journal; use the logging convention of your operating system. A message reported as locally accepted only proves that Postfix queued it. Confirm the upstream SMTP response and the eventual queue status before declaring success.
Test the security boundary
- From the server itself, confirm that the application can submit mail.
- From an unapproved remote address, confirm that the SMTP service is not reachable when it is configured loopback-only.
- If a controlled remote submitter is required, test that its fixed address is accepted while an unlisted address is rejected.
- Send to a recipient domain outside your local destinations and verify that the queue record shows the configured relayhost, not a direct MX connection.
- Inspect the log for a successful TLS handshake and SASL authentication without exposing the password.
- Test a deliberately unauthorized relay attempt in a safe environment and verify that
reject_unauth_destinationblocks it.
Troubleshoot by symptom
“Connection refused” or no listener from the application
- Run
postconf inet_interfacesand confirm that the application connects to a configured loopback address. - Check that Postfix is running and that
postfix checkreports no configuration error. - If the application runs in a container or another host, loopback inside that environment may not be the Postfix host. Use a fixed, restricted address and an appropriate firewall rule instead of exposing all interfaces.
Authentication fails at the relay
- Confirm that the map key exactly matches the bracketed relay host and port in
relayhost. - Re-run the map-generation command after editing the source file.
- Check file ownership and permissions, and verify that the client-side SASL mechanism required by the provider is installed.
- Make sure the account is permitted to use the envelope sender and that the provider has not revoked or limited the credential.
TLS negotiation fails
- Check whether the endpoint expects STARTTLS or implicit TLS.
- For STARTTLS, remove wrapper mode and use the provider’s documented submission port with
smtp_tls_security_level = encrypt. - For implicit TLS, use wrapper mode and the provider’s implicit-TLS port.
- Review the Postfix version and system trust store if certificate validation errors appear.
Mail remains queued
- Read the queue ID’s latest log entry for DNS, connection, TLS, authentication, and provider response errors.
- Check network egress rules, DNS resolution, provider rate limits, and message-size restrictions.
- Do not repeatedly flush the queue while the underlying error is unknown; repeated retries can obscure the original failure and may trigger provider throttling.
The host appears to be an open relay
- Immediately restrict
inet_interfacesto loopback or the required private interface. - Review
mynetworksfor broad subnets and remove networks that do not need submission access. - Restore
reject_unauth_destinationin the appropriate relay restriction parameter. - Inspect logs for unauthorized attempts and rotate relay credentials if they may have been exposed.
Operational checklist
- Use a fully qualified
myhostnameand an intentional sender domain. - Bind the listener to loopback when all applications are local.
- Use
mynetworks_style = hostor an explicit minimal address list. - Route remote mail through a named relayhost when the server is not prepared for direct delivery.
- Require TLS and select STARTTLS or wrapper mode to match the provider.
- Store SASL credentials in a protected map, compile the map after every change, and never log the secret.
- Keep
reject_unauth_destinationafter the appropriate permit rules. - Validate with
postconf,postfix check, a reload, one controlled message, and log inspection. - Record the operating system and Postfix version with any future incident report.
When direct delivery is justified
Direct delivery can be appropriate when you control a stable public address, authoritative DNS, reverse DNS, sender authentication records, reputation monitoring, bounce processing, and rate management. It is a different operational commitment from configuring a send-only relay client. If those controls are not already in place, an authenticated upstream relay reduces the number of variables and keeps Postfix focused on local queueing and submission.
The Bottom Line
A secure send-only Postfix server is a local submission endpoint, not an Internet-facing mail exchanger: keep its listener and trusted networks narrow, send remote mail through an authenticated TLS relayhost, preserve reject_unauth_destination, and verify actual upstream delivery in the logs.
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