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Postfix troubleshooting becomes much faster when you follow one message through the system instead of changing settings at random. Record the installed Postfix version, submit one controlled message, capture its queue ID, and read every log line for that ID. The failure will normally belong to one stage: local submission, queueing, DNS or route lookup, TCP connection, TLS negotiation, SASL authentication, relay authorization, or acceptance by the recipient’s server.
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This guide shows how to identify that stage, collect safe evidence, interpret deferred and bounced mail, and choose a narrowly scoped fix for a Linux VPS. Keep the original SMTP reply and enhanced status code; they are more useful than a generic statement that “Postfix failed.”
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Do not begin by flushing the entire queue or repeatedly retrying delivery. Those actions can create duplicate messages, trigger rate limits, and destroy the sequence you need to diagnose the problem. First choose one test sender and one recipient, note the exact UTC time, and send a single controlled message.
- Record the sender, recipient, VPS hostname, destination domain, and whether the message is submitted locally, through the submission service, through a smarthost, or directly to the recipient domain’s MX.
- Submit one short message with a unique subject such as
postfix-test-2026-10-15-01. - Immediately search the mail log or system journal for the resulting queue ID. A queue ID is the stable identifier that lets you follow one message through enqueueing, connection attempts, retries, and final delivery.
- Save the complete log lines, including the remote SMTP reply, enhanced status code, protocol (IPv4 or IPv6), and timestamp.
Postfix records SMTP problems and transactions through syslogd or postlogd. On many Linux distributions the file is /var/log/mail.log or /var/log/maillog; systems using systemd may expose the same records through the journal. Use the path your distribution configured rather than assuming one filename.
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Useful log searches
For a traditional mail log, search by queue ID, address, or time window:
grep 'QUEUE_ID' /var/log/mail.log
grep -E '[email protected]|[email protected]' /var/log/mail.logFor a journal-managed system, restrict the query to the mail service and the relevant period:
journalctl --since '10 minutes ago' -u postfix
journalctl --since '2026-10-15 12:00:00' | grep 'QUEUE_ID'Replace QUEUE_ID with the real identifier. Preserve the original lines before making a change; a later retry may produce a different response and obscure the first failure.
Classify the message state before changing configuration
The queue state tells you whether Postfix still owns the delivery attempt or has declared it permanently unsuccessful. It does not, by itself, identify the underlying cause.
Queued or deferred
A queued message remains on the server for later processing. “Deferred” generally means a temporary condition, such as a timeout, a 4xx response, a temporarily unavailable DNS result, or a TLS requirement that could not be met on that attempt. Read the reason after the queue ID; do not infer that every deferred message has the same cause.
Bounced or permanently failed
A bounce indicates a permanent failure was reported to the sender or otherwise recorded as final. Common categories include an invalid recipient, a policy rejection that will not be retried, an authentication or authorization failure, or a configuration error that must be corrected before another attempt. The SMTP status code and enhanced code determine which category applies.
Corrupt queue files
Postfix can move corrupt message files to a corrupt queue for inspection. If the log identifies corruption, preserve the file and its metadata for analysis rather than repeatedly forcing delivery. A corruption report is distinct from a remote recipient rejection.
Inspecting the queue
Use the queue listing command to see message IDs, sender, recipients, and age:
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postqueue -pFor one message, inspect its queue metadata and content only when you have an operational reason to do so. Avoid bulk deletion while diagnosing. Commands that flush or remove messages affect every queued delivery unless you explicitly target a message.
Confirm the installed Postfix version and effective settings
Postfix parameters, defaults, and available features vary by release and by distribution package. Capture the version before copying advice from another system.
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postconf mail_version
postconf -npostconf -n prints the non-default configuration that is currently effective, which is more useful than pasting an entire sample configuration. Also note whether the VPS package separates the main service from a submission service and whether a local policy framework modifies delivery.
Check a parameter without guessing its source
Query individual values when investigating a suspected setting:
postconf myhostname
postconf mydestination
postconf relayhost
postconf inet_protocols
postconf smtp_tls_security_level
postconf smtpd_tls_security_level
postconf smtp_sasl_auth_enable
postconf smtpd_sasl_auth_enableCompare each value with the documentation for your installed release, especially when a parameter came from a guide written for a different version. A setting that exists in one release may be unavailable, have a different default, or interact with another parameter.
Find the failing stage
Use the first stage that fails as your boundary. A later setting cannot repair an earlier failure: SASL credentials cannot fix a missing MX record, and a certificate change cannot fix a blocked TCP port.
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|---|---|---|
| Local submission | The client cannot hand the message to Postfix, or Postfix rejects it before a queue ID appears. | Submission service, local permissions, sender restrictions, message size, and the exact SMTP reply. |
| Queueing | A queue ID exists, but the message remains queued or is marked deferred. | Queue status, retry reason, transport selection, and whether the message file is intact. |
| DNS or route lookup | Logs mention lookup failure, no MX, temporary name resolution failure, or an unusable route. | MX, A, and AAAA answers from the VPS resolver and the selected next hop. |
| TCP connection | Connection timeout, refusal, unreachable network, or an inability to connect to the intended port. | Firewall, provider egress policy, destination port, route, and both address families. |
| TLS negotiation | STARTTLS is absent, handshake fails, certificate validation fails, or a required security level cannot be met. | Advertisement, certificate name and chain, trust store, protocol policy, and TLS logs. |
| SASL authentication | The relay returns an authentication error, or the client never offers an accepted mechanism. | Credential map, lookup permissions, username format, mechanisms, and TLS requirement. |
| Relay authorization | The server accepts a connection but rejects the sender as unauthorized to relay. | Whether authentication occurred on the correct service and whether relay restrictions permit the authenticated client. |
| Recipient acceptance | The remote host returns a 4xx or 5xx response after SMTP commands. | Recipient validity, sender authentication, policy, reputation, and the exact remote explanation. |
Check DNS and transport routing from the VPS
Test name resolution on the VPS itself. A result from your laptop does not prove that the VPS resolver can answer reliably or that it returns the same address family.
dig MX example.org
dig A example.org
dig AAAA example.orgReplace example.org with the actual recipient domain. Confirm that the resolver returns the destination’s MX records and that each selected host has usable A or AAAA records. A temporary resolver failure can produce a defer, while a definitive absence of a usable destination can produce a permanent error or a routing failure, depending on the circumstances.
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Postfix has resolver controls that affect how it handles DNS responses. DANE and TLSA validation additionally depend on DNSSEC validation. If the log points to DNSSEC, TLSA, or a resolver validation problem, investigate the resolver and validation chain rather than changing SMTP credentials.
Test the actual next hop and both address families
From the VPS, test the configured relay or destination on the port Postfix is using. A timeout, refusal, or unreachable route is a network or provider-policy problem before it is a Postfix policy problem.
nc -4 -vz smtp.example.net 25
nc -6 -vz smtp.example.net 25Use the destination and port shown in the log. Port 25 is commonly used for server-to-server delivery; a relay may require another port. Do not assume that a successful IPv4 test proves IPv6 works. If IPv6 is enabled in Postfix but the VPS has an incomplete IPv6 route, delivery can fail or select an unreachable address before an IPv4 retry.
Check local firewall rules, the VPS provider’s outbound SMTP policy, and any network security group. A provider block often appears as a timeout or immediate refusal. In that case, changing main.cf will not open the path; request removal of the restriction or use an authorized relay if that is your operating requirement.
Diagnose STARTTLS and certificate failures
STARTTLS upgrades an SMTP connection to an encrypted session. Encryption protects SMTP data and SASL authentication, but Postfix can be configured either to try TLS opportunistically or to require it.
Verify that the server advertises STARTTLS
For inbound submission, inspect the service that clients actually use, not only the public SMTP listener. The service may have a separate master.cf entry and different smtpd_ parameters. If STARTTLS is not advertised, check that TLS is enabled on that service and that the certificate and key files are readable by the Postfix process.
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The certificate name must match the hostname the client connects to, and the chain must lead to a trusted issuer for the validating client. An expired certificate, missing intermediate certificate, wrong hostname, unreadable private key, or an incomplete trust store can all stop a strict handshake. Read the TLS error in the Postfix log and in the connecting client’s output; do not treat every certificate error as a cipher mismatch.
Use temporary, targeted TLS logging
Postfix provides TLS logging levels ranging from handshake summaries to detailed negotiation and protocol traces. Increase the relevant client or server TLS log level only long enough to reproduce one message, collect the queue-ID-linked evidence, and then restore the normal level. Detailed traces can expose protocol metadata and create large logs.
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For outbound delivery, inspect the smtp_ TLS parameters. For inbound SMTP or submission, inspect the smtpd_ parameters. A client-side setting does not repair a server-side certificate, and vice versa.
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Understand opportunistic versus required TLS
With opportunistic TLS, Postfix attempts STARTTLS when the peer advertises it. If the handshake fails and policy permits, it may retry without TLS. That behavior can result in delivery succeeding without encryption, or in a later defer if the peer becomes unavailable. A strict TLS policy refuses to proceed when the required encryption or certificate conditions are not met, so delivery is deferred until the requirement can be satisfied. Choose the policy based on the security requirement of the specific connection; do not make every destination strict merely to hide a certificate problem.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Separate SASL authentication from relay authorization
SASL answers “who is this client?” Relay authorization answers “is this authenticated or otherwise permitted client allowed to send to remote destinations?” They are related but occur on different sides and often use different parameter families.
Inbound client authentication
For users submitting mail to your VPS, inspect the submission service and its smtpd_sasl_* settings. Confirm that:
- the service actually enables SASL;
- the configured authentication backend or password map contains the expected username;
- the Postfix process can read the map and its parent directories;
- the client’s username format matches the backend;
- the offered mechanism is allowed; and
- TLS is required before credentials are accepted when your security policy calls for it.
A permission error while opening a password map can look like a bad password from the client. Check the Postfix log for the map name, lookup result, and filesystem error before resetting credentials.
Outbound relay authentication
When Postfix sends through an upstream relay, inspect the smtp_sasl_* settings and the relay host. Verify that the credential map uses the exact relay hostname and expected username format, that the map has been compiled or generated as required by your package, and that the Postfix process can read it. Confirm the permitted mechanisms and whether the relay requires TLS before AUTH.
Do not enable outbound SASL merely because inbound clients cannot authenticate. Those are separate paths. Likewise, adding a client to a relay restriction list does not create a valid SASL login.
Interpret authentication replies
A reply such as “authentication failed” points to credentials, mechanism negotiation, map access, or a TLS prerequisite. A reply that says the client is “not authorized to relay” indicates authorization policy, even when authentication succeeded. Follow the queue ID and identify whether the rejection came from your Postfix instance or from the upstream relay.
Read remote SMTP replies literally
The receiving server normally states the immediate reason for rejection. Preserve the complete reply and enhanced status code, including any explanatory text or policy reference.
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A 4xx response asks the sending server to try later. The cause may be temporary throttling, greylisting, a busy destination, a transient DNS condition, or a temporary policy decision. Postfix keeps the message queued and retries according to its delivery schedule. Repeated manual flushes do not remove the underlying condition and can increase pressure on the destination.
Permanent 5xx responses
A 5xx response generally requires a correction before another attempt. Recipient-not-found errors call for checking the address and destination domain. Sender-authentication or policy errors call for examining identity, authorization, reverse-DNS expectations, message policy, or the relay’s requirements. Do not rewrite a specific remote response as a generic Postfix defect.
Reputation and policy rejections
A remote host may reject a message because of IP reputation, rate, missing or inconsistent sender authentication, policy rules, or a blocklist. The SMTP text may identify which condition applies. Fix the named identity or policy issue, reduce uncontrolled retries, and test one message after the change. A new VPS address can have a different reputation from a previous server, so copying the old Postfix configuration does not guarantee acceptance.
Use controlled remediation instead of broad changes
Choose a fix by the stage it addresses and by its blast radius. The following comparisons help prevent a security or compatibility regression.
| Remediation | Addresses | Security and compatibility considerations |
|---|---|---|
| Correct MX, resolver, or route configuration | DNS and connection failures | Usually narrow in scope; verify the VPS resolver and both IPv4 and IPv6 paths. |
| Repair certificate, trust chain, or hostname | TLS handshake and validation failures | Preserves encryption requirements; confirm the installed Postfix release and service-specific files. |
| Adjust opportunistic versus strict TLS policy | Peers that do or do not meet TLS requirements | Changing from required to opportunistic can permit unencrypted fallback; make that trade-off explicit. |
| Fix SASL map, permissions, or mechanism | Authentication failures | Limit the change to inbound submission or outbound relay as appropriate; protect credentials with TLS. |
| Use an authorized smarthost | Direct-to-MX blocks, provider restrictions, or reputation problems | Changes the delivery path and introduces relay credentials and policy dependencies. |
| Change IPv4/IPv6 selection | One address family has a broken route or reputation | Scope the change deliberately and repair the disabled path rather than hiding a persistent network defect. |
After one change, submit one new controlled message and follow its new queue ID. Compare the exact stage and response with the original. If the result changed, keep the evidence; if it did not, revert the change before trying a different stage.
Common symptom-to-action branches
Mail is stuck in the queue with a timeout
- Find the queue ID and the destination host in the timeout line.
- Resolve that host from the VPS and test the same port over IPv4 and IPv6.
- Check firewall and provider egress rules.
- Only after connectivity works, investigate TLS or recipient policy.
Local delivery works, but Gmail or another remote domain does not
Local delivery proves that submission and local transport work; it does not prove DNS, outbound TCP, TLS, IPv6, relay authorization, or remote acceptance. Follow the first remote-delivery log line, identify the destination and address family, and read the remote response. A provider block, broken IPv6 route, missing MX result, or reputation policy can each produce this pattern.
Clients report SMTP authentication failure
- Confirm they are connecting to the intended submission service and port.
- Check whether STARTTLS is required and actually advertised.
- Verify username format, mechanism restrictions, password-map contents, and map permissions.
- Read the server-side SASL log entry; a filesystem lookup error is not the same as an incorrect password.
STARTTLS fails immediately
Determine whether the failure is on inbound submission or outbound delivery. Check advertisement, certificate name, expiration, chain, private-key permissions, trust validation, and the configured security level. Temporarily collect a detailed TLS trace for one reproduction, then lower logging again.
The relay says “not authorized” after a successful login
Authentication and relay permission are separate checks. Confirm that the authenticated service applies the expected relay restrictions, that the sender or destination is permitted, and that the response came from the relay you intended to use rather than from a different next hop.
Finish with a safe verification checklist
- Record the Postfix version and effective non-default configuration.
- Submit exactly one test message and capture its queue ID.
- Save complete log lines and the remote enhanced status code.
- Identify the first failing stage: submission, queue, DNS, TCP, TLS, SASL, authorization, or recipient acceptance.
- Check both IPv4 and IPv6 whenever both are enabled.
- Validate DNSSEC-dependent behavior when using DANE or TLSA.
- Apply one reversible change with the smallest practical scope.
- Retest with one message and compare the new queue-ID path.
- Restore temporary debug logging and remove test credentials or files that are no longer needed.
This method leaves you with an auditable explanation of what failed and why the fix worked, rather than an unverified collection of configuration changes.
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