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Configure Postfix as a send-only server by binding its SMTP listener to localhost, allowing local applications to submit messages, routing non-local mail through an authenticated upstream relay, and enforcing TLS plus relay authorization. This setup queues outbound mail but does not make the server an Internet mailbox or an open relay.
What “send-only” means in Postfix
A send-only Postfix host accepts mail from applications on the same machine (or from explicitly permitted application hosts), places it in the queue, and forwards remote recipients through an SMTP relay. It is not intended to receive Internet mail for a domain, maintain user mailboxes, or accept arbitrary mail from the public network.
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- Who can connect to Postfix: listener interfaces and trusted client networks.
- Where Postfix can relay: recipient authorization and the upstream
relayhost.
Restricting the listener alone is not a substitute for a relay policy. Keep a destination check such as reject_unauth_destination even when the daemon is bound only to loopback.
Choose the outbound delivery model
Before editing files, decide whether Postfix will deliver directly to recipient domains or submit every remote message to a provider. A server without a stable public IP address and carefully maintained DNS normally benefits from an authenticated relay.
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| Model | How it works | Operational implications |
|---|---|---|
| Authenticated relay (recommended for most send-only hosts) | Postfix connects to one provider endpoint, authenticates, negotiates TLS, and submits all remote mail there. | The provider handles much of the receiving-server compatibility, reputation, and outbound routing. You must protect credentials and follow its rate and policy limits. |
| Direct delivery | Postfix looks up recipient-domain MX records and opens SMTP sessions to those destination servers. | You must manage public DNS, reverse DNS, IP reputation, TLS behavior, throttling, bounces, and remote-server acceptance. A residential, dynamic, or shared IP is often unsuitable. |
The remainder of this guide uses an authenticated relay. If you choose direct delivery, leave relayhost empty and verify that your public identity, DNS, reverse DNS, firewall, and reputation are appropriate before sending production mail.
Prerequisites and package installation
Collect the values you will need
- The server’s fully qualified hostname, such as
mailer.example.net. - The domain that should appear in locally generated envelope and header addresses.
- The relay provider’s SMTP submission hostname and port.
- The provider’s required TLS mode: STARTTLS or implicit TLS (sometimes called submissions or wrapper mode).
- An SMTP username and credential, or the provider’s required token or application password.
- The local applications or fixed remote addresses that are allowed to submit mail.
Provider requirements take precedence over examples in this article. Some services require a verified sending domain, an application-specific password, or a modern authentication method rather than a normal account password.
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Install the Postfix package using your operating system’s package manager. Package names and setup dialogs differ between distributions; choose an Internet-site or satellite-style option only when its routing behavior matches your plan. After installation, record the version:
postconf mail_versionKeep the version with your deployment notes. Parameter defaults, service names, and available SASL mechanisms can vary between releases.
Set the server identity
Postfix uses several identity parameters for locally generated messages and for SMTP greetings. Edit /etc/postfix/main.cf (the path is standard on most Linux installations) and review these values:
myhostname = mailer.example.net
mydomain = example.net
myorigin = $mydomain
mydestination = $myhostname, localhost.$mydomain, localhostSet myhostname to the host’s fully qualified name, not a short name. Set myorigin to the domain that should be appended to local addresses. Keep the identity consistent with the name and domain accepted by the upstream relay. The exact mydestination list depends on whether the machine should deliver mail addressed to itself; it does not turn the host into an Internet-facing mailbox when the SMTP listener is restricted to loopback.
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Check the effective values rather than relying only on what appears in the file:
postconf myhostname mydomain myorigin mydestinationBind SMTP to local interfaces
Use loopback-only for local applications
For software running on the same machine, use:
inet_interfaces = loopback-only
mynetworks_style = hostinet_interfaces = loopback-only makes Postfix listen on loopback addresses instead of public network interfaces. mynetworks_style = host trusts only the local machine. This combination prevents ordinary remote hosts from reaching the SMTP service and avoids accidentally trusting an entire subnet.
After changing inet_interfaces, reload Postfix and confirm the listening sockets with the tools available on your system, for example ss -ltnp. You should see loopback addresses rather than a wildcard such as 0.0.0.0:25 or [::]:25.
Permit a separate application host only when required
If an application runs on another fixed server, bind Postfix to an interface that can reach that server and define a narrowly scoped network list instead of opening SMTP globally:
mynetworks = 127.0.0.0/8, [::1]/128, 192.0.2.25/32Replace 192.0.2.25/32 with the submitting host’s real fixed address. A single-host CIDR (/32 for IPv4) is safer than trusting a broad private subnet. Protect the path with a firewall and, where practical, use SMTP authentication for the remote application instead of relying solely on network trust.
Do not expose port 25 to the Internet merely because a local application needs SMTP. If a remote client must submit mail, expose only the required interface and port, restrict source addresses, and require authentication.
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Configure the upstream relay
Set the relay host and port
Add the provider’s submission endpoint to main.cf:
relayhost = [smtp.example-provider.net]:587Square brackets tell Postfix to connect to that exact host without performing an MX lookup for it. Use the port specified by the provider; 587 commonly means submission with STARTTLS, while another provider may require implicit TLS on a dedicated submissions port.
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Postfix routes remote destinations through relayhost. Local destinations that match your local-delivery configuration can still be handled locally. Confirm the result with:
postconf relayhostChoose the correct TLS mode
TLS encrypts the SMTP session and protects SASL authentication data while it is in transit. For STARTTLS, use the provider’s submission port and its documented mode. A typical policy is:
smtp_tls_security_level = encryptThis requires TLS before the SMTP client proceeds. Do not copy an implicit-TLS example onto a STARTTLS port. For a provider that explicitly documents wrapper mode (implicit TLS), the pattern is:
relayhost = [smtp.example-provider.net]:465
smtp_tls_security_level = encrypt
smtp_tls_wrappermode = yesWrapper mode starts TLS immediately when the TCP connection opens. STARTTLS begins as plain SMTP and upgrades after the server advertises the extension. Use one mode, not both, according to the provider’s instructions.
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For stricter deployments, validate the relay certificate and require the expected security level. Avoid disabling certificate verification just to make a connection succeed; diagnose hostname, trust-store, or clock problems instead.
Enable SMTP-client authentication
Create a protected password map
Enable the SMTP client and point it to a map containing the relay credentials:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymousCreate /etc/postfix/sasl_passwd with the relay key and credential in the format required by the selected map:
[smtp.example-provider.net]:587 relay-user@example.net:REPLACE_WITH_SECRETThe key must match the bracketed hostname and port in relayhost. If you use a different port or wrapper-mode endpoint, change the key accordingly. Do not publish a real password in a configuration example, shell history, ticket, or log.
Build the map and lock down permissions
Set ownership and permissions so only the Postfix administrator can read the source file, then generate the database:
chown root:root /etc/postfix/sasl_passwd
chmod 600 /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd
chmod 600 /etc/postfix/sasl_passwd.dbhash: is common, but distributions may package another map backend. Use the backend installed on your system and run its corresponding map-generation command after every edit. Remove or rotate obsolete credentials when changing providers.
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Some providers no longer accept basic username-and-password authentication. In that case, use the provider’s supported token or application password and confirm that Postfix’s SASL mechanisms match the service. A successful TCP connection without successful authentication is not a configured relay.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep relay authorization closed
Postfix must reject unauthorized destinations even on a send-only host. On Postfix 2.10 and later, put the policy in smtpd_relay_restrictions:
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smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destinationpermit_mynetworks allows only the hosts in mynetworks; permit_sasl_authenticated allows clients that authenticated successfully; reject_unauth_destination blocks relaying to destinations that Postfix is not authorized to serve. Keep the rejection condition. Never replace it with a blanket permit.
If only local processes submit mail, loopback-only listening and host-only trust may be enough. If remote clients submit, require SASL or define a tightly scoped network list. Review older configurations that place relay restrictions only in smtpd_recipient_restrictions; the modern parameter is smtpd_relay_restrictions, although a valid legacy policy may still exist for compatibility.
Inspect the effective policy:
postconf smtpd_relay_restrictions mynetworks smtpd_recipient_restrictionsValidate, reload, and send a controlled message
- Review effective settings. Run:
postconf myhostname relayhost inet_interfaces mynetworks smtp_tls_security_level smtp_tls_wrappermode smtp_sasl_auth_enable smtp_sasl_password_maps - Check the configuration.
postfix checkCorrect every reported file, permission, or directory error before testing delivery.
- Rebuild credentials. Run
postmapagain if the password file changed, and confirm that the generated map exists with restrictive permissions. - Reload Postfix.
postfix reloadSome distributions manage the service through a unit manager; use that system’s reload action when appropriate.
- Submit one test message locally. Use the local
sendmailcommand or the application’s normal SMTP settings. Send to an address you can inspect, not a large list. - Follow the mail log. Find the queue ID and confirm the relay hostname, TLS negotiation, authentication result, upstream response, and final delivery status. Log paths differ by distribution, commonly under
/var/log/maillogor/var/log/mail.log. - Check the queue if delivery is delayed.
postqueue -pA message accepted into the local queue proves only that Postfix received it; it does not prove that the upstream provider accepted or delivered it.
Document the operating system, Postfix version, relay endpoint, TLS mode, and the exact error line when escalating a failure. Those details determine which service command, log location, and parameters apply.
Troubleshoot common failures
Connection refused or timeout
- Confirm that
relayhosthas the correct hostname and port. - Check firewall egress rules and provider restrictions on the selected port.
- Verify DNS resolution and system time; an incorrect clock can break TLS validation.
- Ensure wrapper mode is not enabled on a STARTTLS endpoint, and vice versa.
“SASL authentication failed”
- Check that the password-map key exactly matches the bracketed relay host and port.
- Run
postmapafter editing the source file and reload Postfix. - Verify file ownership and permissions, then confirm the provider expects the supplied credential type.
- Look for account restrictions such as a required application password, token, or verified sender.
TLS certificate or handshake errors
- Make sure the relay hostname matches the certificate name; using an unexpected alias can fail verification.
- Install or update the operating system’s CA trust store and correct the server clock.
- Use the provider’s required TLS mode and minimum version rather than weakening encryption.
“Relay access denied” or unauthorized destination
- Confirm that the message is going to the configured relay and that authentication succeeded.
- Check
smtpd_relay_restrictionsfor an unintended order or a missingpermit_mynetworks. - For a remote submitting host, verify its address is in
mynetworksor that it authenticates.
Mail is accepted locally but remains queued
- Inspect the queue ID’s log entries for repeated connection, TLS, DNS, or authentication errors.
- Check provider rate limits, account suspension, recipient policy, and message-size limits.
- After fixing the cause, allow the normal retry interval or request a queue run with
postqueue -f; do not repeatedly flush a queue while the underlying error remains.
Remote hosts can connect to port 25
- Recheck
inet_interfacesand the actual listening sockets. - Look for an override in
master.cfor a second Postfix instance. - Apply a firewall rule that permits only loopback or the specific submitting addresses.
- Review
mynetworks; a broad subnet or an automatically detected network can create unintended trust.
Security and operational checks
- Keep credentials readable only by the Postfix administrator and rotate them when staff, hosts, or providers change.
- Use encrypted upstream submission and retain certificate verification.
- Limit listener addresses and firewall exposure; send-only does not mean “open SMTP.”
- Keep
reject_unauth_destinationin the relay policy and test from an unauthorized client when possible. - Monitor queue growth, deferred messages, bounce rates, and provider responses.
- Set realistic message-size and rate limits in the application and provider account.
- Separate application sender identities where your provider supports it, so a compromised application can be disabled without replacing every credential.
- Back up configuration without exposing
sasl_passwd; store secrets in an approved secret-management system when available.
Configuration reference
| Parameter | Purpose | Typical send-only value |
|---|---|---|
myhostname |
Fully qualified identity used by Postfix | mailer.example.net |
myorigin |
Domain appended to local addresses | $mydomain |
inet_interfaces |
Interfaces on which SMTP listens | loopback-only |
mynetworks_style |
Automatic trusted-network scope | host |
mynetworks |
Explicit trusted clients | Loopback plus fixed application addresses only |
relayhost |
Upstream SMTP submission endpoint | [smtp.example-provider.net]:587 |
smtp_tls_security_level |
Client TLS requirement | encrypt |
smtp_tls_wrappermode |
Implicit TLS from connection start | yes only when the provider requires wrapper mode |
smtp_sasl_auth_enable |
Enable SMTP-client authentication | yes |
smtp_sasl_password_maps |
Credential lookup map | hash:/etc/postfix/sasl_passwd |
smtpd_relay_restrictions |
Controls who may relay | permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination |
Final verification checklist
- The host has a stable, intended
myhostnameand sender domain. - Postfix listens only on loopback or explicitly required interfaces.
mynetworkscontains no unnecessary subnet or public address.relayhostuses the provider’s exact hostname, port, and TLS mode.- The password map exists, was rebuilt after edits, and is protected with restrictive permissions.
- Relay restrictions include
reject_unauth_destination. postfix checksucceeds and the service has been reloaded.- A controlled message shows successful TLS, authentication, upstream acceptance, and final delivery in the logs.
- Queue, bounce, and deferred-mail monitoring is in place before production use.
With these controls, Postfix performs the narrow role a send-only host needs: local submission, queued outbound delivery, and authenticated forwarding without accepting unsolicited Internet mail or operating as an open relay.
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