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To stop spam being sent through Postfix, lock down relay authorization before adding spam filters. On Postfix 2.10 and newer, put the relay policy in smtpd_relay_restrictions: permit only trusted networks and SASL-authenticated clients, then defer unauthorized destinations with defer_unauth_destination. Keep DNSBL, sender, HELO, greylisting, SPF and content-filter checks in their appropriate restriction lists, and verify the effective configuration with an unauthenticated external relay test, logs and queue inspection.
What “spam through Postfix” usually means
There are two different problems that administrators often call “Postfix spam.” The first is an open relay: an outside, unauthenticated client can submit a message whose destination is not one of your local or explicitly relayed domains. Spammers can then use your server to deliver bulk mail, damaging your IP reputation and filling the queue. The second is abusive or unwanted mail that your server is legitimately accepting, such as messages sent to local users or mail submitted by an authenticated account that has been compromised.
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Relay authorization has to be fixed first. DNS blocklists, sender checks and content filters can reduce unwanted mail, but none of them substitutes for a rule that decides who may relay and where. Postfix’s documented model is that mail is forwarded only from trusted networks, from clients authenticated with SASL, or to domains configured as authorized relay destinations.
The exact parameter names and service layout differ between Postfix and CentOS/RHEL releases. Treat the examples below as a policy structure to adapt, not as a universal copy-and-paste configuration.
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Put relay authorization in the right restriction list
Use smtpd_relay_restrictions on Postfix 2.10 and newer
Postfix introduced smtpd_relay_restrictions in version 2.10 as a safety mechanism against mistakes in smtpd_recipient_restrictions that could turn a server into an open relay. It is the primary modern control for deciding whether a client may relay to a non-local destination.
A conservative baseline in /etc/postfix/main.cf is:
# Relay authorization
smtpd_relay_restrictions = permit_mynetworks,
permit_sasl_authenticated,
defer_unauth_destination
# Keep recipient policy separate and deliberate.
smtpd_recipient_restrictions = permit_mynetworks,
permit_sasl_authenticated,
reject_unauth_destinationThe first list allows clients in mynetworks, allows clients that completed SASL authentication, and defers an attempt to relay somewhere else. The second line is a defensive recipient policy and a place for additional anti-spam checks. Whether you retain the same permits and the final reject in that list depends on your Postfix version and the policy you want; inspect the effective configuration rather than assuming a commented example is active.
Postfix requires the effective relay controls to enforce a reject policy such as reject_unauth_destination (or an equivalent rejection condition). If neither smtpd_relay_restrictions nor smtpd_recipient_restrictions provides that protection, Postfix refuses to receive mail rather than knowingly operate without a relay boundary.
Understand left-to-right evaluation
Restriction lists are processed from left to right. Evaluation stops when a rule returns PERMIT, REJECT or DEFER. That ordering makes exceptions significant: a trusted-network or authenticated-client permit must appear before a later rule intended for untrusted clients, while an overly broad permit placed first can bypass every subsequent rejection.
For example, placing permit or a wide network allowance before relay protection can authorize destinations that should have been refused. Conversely, putting a DNSBL rejection before permit_sasl_authenticated means an authenticated user whose client address appears on that list may be blocked. Decide whether trusted and authenticated clients should bypass each check, then order the rules to match that decision.
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Define the two intended exceptions explicitly
- Trusted networks:
permit_mynetworkscovers addresses listed in Postfix’smynetworkssetting. Keep this network narrowly scoped to your server, application hosts and other systems that you administer. Do not include an entire provider or office range merely for convenience. - Authenticated clients:
permit_sasl_authenticatedallows clients that have successfully authenticated through your configured SASL service. Require authentication on the submission service and protect credentials with the transport security appropriate to your deployment. - Authorized destinations: A domain configured as local or as an explicitly authorized relay destination is not an open-relay case. Review those domain lists whenever you add or remove hosted domains.
Every client that does not match a trusted network or valid SASL session should be unable to use your server to deliver to an unrelated external domain.
Choose defer and reject deliberately
defer_unauth_destination returns a temporary failure for an unauthorized relay attempt. It is the standard safety-oriented action in the modern relay restriction list. reject_unauth_destination rejects the attempt immediately and is commonly used in recipient restrictions, especially on older configurations. Neither action grants relay permission; both prevent an unauthenticated client from relaying to a destination your server is not authorized to serve.
Build a policy that matches your Postfix generation
Modern Postfix configuration
On a current CentOS-style server, begin by identifying the active values, then edit the configuration file that your service actually loads. Keep the relay list short and obvious. Add anti-spam rules only after you can explain which clients each rule should affect.
postconf -nUse the output to check for duplicate settings, unexpected network ranges and an existing recipient policy. In Postfix syntax, a value can span lines when continued with whitespace, as in the example above. Preserve commas and continuation indentation exactly enough for Postfix to parse the list.
Older Postfix releases
Before version 2.10, relay and spam policy were commonly combined under smtpd_recipient_restrictions. The documented safe order places reject_unauth_destination before other checks so that later permits cannot accidentally create an open relay. If you maintain an older CentOS or RHEL installation, check the installed Postfix version and its parameter manual before translating a modern example. Do not assume that copying a newer parameter to an older daemon will work.
Do not use a broad permit as a shortcut
A line such as an unrestricted permit near the beginning of a recipient list defeats the purpose of the later relay reject. The same is true of an overbroad mynetworks value. If an application needs to send mail, give it a fixed address or authenticated account and test only that path.
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Add anti-spam controls without replacing relay protection
Once relay authorization is sound, layer controls according to the traffic you actually receive. Each layer has a different signal and failure mode. Keep the relay decision independent so that a temporary DNS or policy-service problem cannot silently authorize external delivery.
| Layer | What it checks | Where to place it | Main caution |
|---|---|---|---|
| DNSBL | Whether a connecting address appears on a blocklist | A client or recipient restriction, such as reject_rbl_client |
Operators have different listing criteria, terms and availability; verify the service and expect false positives. |
| HELO validation | Whether the client’s greeting is syntactically and operationally acceptable | HELO restriction list, after deciding which clients are exempt | Strict checks can reject older or unusual but legitimate senders. |
| Sender/access tables | Addresses, domains or client patterns that you explicitly allow or deny | Access restrictions using the tables supported by your release | Keep entries documented and remove temporary exceptions. |
| Greylisting | Whether a sender retries after an initial temporary failure | An external policy service or milter, not a replacement for relay authorization | It delays legitimate first deliveries and requires operational monitoring. |
| SPF policy service | Whether a sender is authorized for the envelope domain | An external policy mechanism integrated with Postfix | SPF is a sender-domain signal; it does not decide whether your client may relay. |
| Content filtering | Message body, attachments and other content indicators | Before-queue or after-queue filtering, depending on the product and policy | Filters consume resources and can quarantine or defer legitimate mail. |
DNSBL checks
The CentOS example in the Postfix documentation demonstrates reject_rbl_client. A DNSBL can reject known abusive client addresses before a message is accepted, but the list’s operator controls its data and terms. Confirm that queries are permitted, understand how listings are removed, and provide a way to investigate a legitimate sender who is listed. Never treat a DNSBL result as a substitute for defer_unauth_destination or reject_unauth_destination.
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HELO, client and sender checks can stop malformed or suspicious SMTP sessions. Place them where their exceptions are intentional. If authenticated submission clients should be allowed to use an internal hostname or a sender address that would be suspicious on the public MX, permit those clients before the relevant check. If you want authenticated accounts subject to the same anti-abuse controls, put the check first and accept the resulting support burden.
Greylisting and SPF services
Postfix documents greylisting and SPF policy mechanisms separately from its built-in access controls. They are policy services, often external to the core daemon, and their availability affects mail flow. Start in a monitoring or limited-scope mode if your integration supports one. Record which clients are exempt, what a temporary failure looks like in the logs and how to disable the service without removing the relay boundary.
Content filters
A content filter can operate before a message enters the queue or after queue acceptance. Before-queue filtering can reject during SMTP; after-queue filtering can accept first and then quarantine, re-route or discard according to its policy. Whichever model you use, reserve resources for ordinary delivery and watch queue growth. A filter that is unavailable should not turn an otherwise protected relay into an accepting path.
Harden the CentOS/RHEL host around Postfix
Limit network exposure
Red Hat’s RHEL 9 security guidance notes that Postfix listens on the local loopback address by default. Verify the effective inet_interfaces value instead of assuming that the daemon is publicly reachable. If the server is not an Internet-facing mail exchanger, bind only the interfaces that need SMTP and keep the firewall closed to every other source.
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If the host receives mail directly, expose only the SMTP service required for that role. Treat the public port-25 service and the authenticated submission service as separate trust zones: the former accepts inbound mail for hosted domains, while the latter should require SASL authentication and appropriate transport security before allowing relay.
Require authentication on submission
Authentication is an authorization mechanism, not merely a convenience for mail clients. Audit which SASL mechanisms are enabled, where credentials are stored and how failed logins are logged. Disable or restrict anonymous submission. When an account is suspected of compromise, remove or rotate its credentials and inspect its recent message activity and queue entries.
Apply rate and message-size limits
SMTP restrictions cannot prevent every abuse pattern. A valid authenticated account can still submit a burst of messages, and an inbound sender can consume resources with oversized messages or many simultaneous connections. RHEL guidance recommends strict rate and message-size limits to mitigate flooding. Set limits appropriate to your users and applications, then watch for legitimate workloads that need a documented exception.
Separate queue and log monitoring from SMTP decisions
Enable the service’s normal logging and review it for repeated authentication failures, bursts of rejected recipients, unusual authenticated senders and repeated deferred deliveries. Inspect the queue with:
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Verify the relay policy before declaring the server fixed
Use a controlled test account or an external test host that you own. Do not probe third-party systems without authorization. Exact service names and test commands vary by Postfix and CentOS/RHEL release, but the validation sequence is consistent.
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- Inspect effective settings. Run
postconf -nand compare the output with the intended values forsmtpd_relay_restrictions,smtpd_recipient_restrictions,mynetworks, SASL and interface exposure. Check for an unexpected permit, an overly broad network and a missing unauthorized-destination rule. - Map the trust boundaries. Write down which source addresses are in
mynetworks, which submission clients authenticate and which domains are local or authorized relay destinations. Anything outside those sets should not relay externally. - Run an unauthenticated external test. Connect from outside the trusted networks, begin an SMTP session and issue a controlled
RCPT TOfor a destination outside the server’s local domains. The server should reject or defer the recipient according to the configured relay policy. Stop before sending a message if the response shows that the destination would be accepted. - Test permitted delivery separately. From an approved network, deliver to an authorized destination. Repeat from an authenticated submission client to a non-local destination. These tests confirm that the relay lockout did not break the two intended exceptions.
- Read the logs for each test. Match the timestamp, client address, authentication identity, recipient and final SMTP result. A rejection caused by a DNSBL or sender check is different from a relay-policy refusal; identify the rule before changing it.
- Inspect the queue after testing. Run
mailqand confirm that the controlled test did not leave unexpected messages. Continue watching the queue for repeated unauthorized attempts, bursts or deferred mail after the change.
Troubleshoot common failure patterns
An external client can relay
First inspect postconf -n for a broad mynetworks entry or a permit earlier in the active restriction list. Confirm that the service loaded the file you edited and that the effective relay list contains permit_mynetworks, permit_sasl_authenticated and an unauthorized-destination defer or reject. Re-run the external RCPT TO test after correcting the order.
Legitimate authenticated users are rejected
Check whether the client is actually authenticated before the recipient restriction runs. Review the SASL log entry, the account identity and the submission service’s policy. If a DNSBL, HELO or sender rule precedes permit_sasl_authenticated, decide whether that rule should apply to authenticated users; move the permit only if bypassing the check is an intentional, documented exception.
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Inbound mail to hosted domains is refused
Verify that the recipient domain is configured as local or as an authorized relay destination and that the test address exists according to your mailbox setup. Do not “fix” a local-domain problem by adding a broad permit. The relay policy should continue to reject unrelated external destinations.
The queue grows after enabling a filter or DNSBL
Compare the queue entries and log reasons. A policy service timeout, DNS lookup failure, filter backlog or remote delivery problem can all produce deferrals. Temporarily narrow or disable the failing additional layer according to your change procedure, but leave the relay authorization controls in place. Re-enable the layer only after confirming its availability and expected response.
A listed sender claims the DNSBL result is wrong
Record the client address, listing name, timestamp and SMTP response. Check the list operator’s removal and dispute process, then decide whether a narrowly scoped exception is safer than disabling the list. Avoid permanent broad allowlists created to solve one incident.
Choose a policy for the server’s actual role
Local-only application host
If the machine sends mail only for local applications, keep Postfix bound to loopback where practical, limit mynetworks to the application addresses and do not expose a public submission service. Test that an external client cannot connect or relay, while the application can deliver to the destinations it requires.
Authenticated submission server
Separate authenticated submission from inbound MX traffic. Require SASL on the submission path, keep permit_sasl_authenticated in the deliberate exception position and impose per-client or account rate limits appropriate to your users. Monitor authentication failures and sudden changes in recipient volume.
Inbound MX for hosted domains
Accept unauthenticated SMTP only for domains the server is responsible for. Use recipient, HELO, client and DNSBL checks for inbound abuse, but retain the relay restriction so an Internet sender cannot choose an unrelated destination. Test both a valid hosted recipient and an external recipient from the same unauthenticated connection.
Operate the configuration safely over time
- Record every exception: document why an address is in
mynetworks, why a domain is relay-authorized and who owns the exception. - Change one policy layer at a time: otherwise a new DNSBL, filter and rate limit can make a rejection impossible to attribute.
- Validate after every reload or package update: inspect
postconf -n, perform the controlled relay test and check the queue. - Review exposure periodically: remove unused interfaces, firewall rules, SASL accounts and old hosted domains.
- Keep a rollback: save the known-good configuration and a record of the previous restriction order before changing policy.
The durable defense is a short, auditable relay policy with explicit trusted-network and authenticated-user exceptions, followed by anti-spam layers that are tested independently. Host exposure, rate limits, queue review and logs close the gaps that SMTP restriction lists cannot address on their own.
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