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Postfix can accept a message locally and still fail to deliver it. The reliable fix starts with the message’s queue ID and the exact SMTP or network error, then follows that message through the logs and queue. This guide shows how to identify the failure, distinguish a temporary deferral from a permanent bounce, correct relay and TLS settings, work around VPS port-25 blocks, and validate DNS and sender authentication without accidentally creating an open relay.
Start with the queue ID, not the symptom
“Postfix is not sending” describes several different outcomes. A local application may have handed the message to Postfix successfully, while Postfix is waiting for a retry, has rejected the message, or has delivered it to a remote server that later placed it in spam. Treat each test message as a traceable transaction.
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- Check the service:
systemctl status postfix. If it is stopped or failed, inspect the service output and system journal before changing SMTP settings. - Submit one test message with a distinctive subject and a destination mailbox you control. For a Postfix installation that provides the sendmail-compatible command, an example is:
printf 'Subject: Postfix diagnostic testnnQueue tracing test.n' | sendmail -v you@example.net - Immediately note the queue ID shown in the mail log. A successful local submission only proves that Postfix accepted the message into its own pipeline; it does not prove remote delivery.
- Follow that queue ID until the log records a delivery, a permanent bounce, or a temporary deferral.
Postfix documents that problems and transactions are written to the system logging service. Ubuntu identifies /var/log/mail.log as the main Postfix log, with errors and warnings also available in /var/log/mail.err and /var/log/mail.warn. On distributions that route mail events to systemd or another syslog location, use the equivalent journal or mail facility.
Watch the logs while reproducing the failure
Open a second terminal and follow the relevant log while sending exactly one test message:
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tail -f /var/log/mail.logFilter mentally, or with your system’s log tools, for the queue ID and for lines containing status=. The final status is more useful than an application’s generic “send failed” message because it includes the remote SMTP response or the local connection error.
What the queue status means
| Status | Meaning | What to do |
|---|---|---|
| Active | Postfix is processing the message. | Keep watching the queue ID and log; an active message may become delivered or deferred. |
| Deferred | A temporary failure prevented delivery. | Read the reason after status=deferred; correct the underlying DNS, network, TLS, rate-limit, or remote-server issue and allow Postfix to retry. |
| Bounced | Delivery failed permanently, or a policy rejected the message. | Use the SMTP response to correct the address, authorization, identity, or policy problem before sending again. |
| Delivered | The recipient server accepted the message. | Postfix transport succeeded; investigate recipient filtering or spam placement separately. |
Use postqueue -p or mailq to list queued messages. Match the queue ID in that output with the log lines. Postfix queue services retain status for delayed, failed, and successful deliveries, so an apparently successful local command should never be your only verification.
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status=deferred
Deferred means Postfix received a temporary failure and normally keeps the message for another delivery attempt. The cause is in the text of the same log entry. Frequent categories are a destination DNS problem, an unreachable server, a remote rate limit, or a TLS negotiation failure.
- Confirm that the recipient domain has usable MX records and that your VPS can resolve them.
- Check outbound connectivity to the selected mail host and SMTP port.
- Look for a remote response such as a temporary policy or rate-limit code; repeatedly forcing retries will not remove a remote limit.
- For TLS errors, verify the relay hostname, certificate name, trust chain, and the configured security level.
Do not delete a deferred message merely because it is still queued. Preserve its queue ID and error text; those identify whether the next action belongs in DNS, the firewall, Postfix TLS settings, or the recipient’s policy.
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relay access denied
Relay authorization is intentional. Postfix should relay for explicitly trusted networks, authenticated clients, or destinations you have configured as permitted. The relevant controls include mynetworks, SASL authentication, and smtpd_relay_restrictions.
This error commonly appears in two situations:
- An application connects to the local SMTP service from an address not included in
mynetworksand does not authenticate. - Your server is attempting to use another SMTP server as a relay without supplying the credentials or authorization that server requires.
Review which service is producing the error. For inbound submissions to your VPS, permit only the networks and authenticated users you actually operate. For outbound submission to a relay provider, configure that relay’s required SASL mechanism and credentials. Never “fix” the message by allowing every network or by removing destination restrictions: a permissive configuration can turn the VPS into an open relay used to send junk mail.
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A TLS connection can fail even when the relay host is reachable. The hostname in relayhost must match the name covered by the relay’s certificate, and the VPS must trust the issuing chain. A certificate that is expired, self-signed, issued for a different name, or rejected by the selected TLS security level can leave the message deferred.
- Read the complete TLS error in the mail log, including the hostname Postfix tried to verify.
- Use the provider’s documented SMTP hostname rather than an IP address when certificate verification is required.
- Check that the VPS has current CA certificates and an accurate system clock.
- Match
smtp_tls_security_leveland any provider-specific TLS requirement to the relay’s instructions. Do not disable verification as a permanent workaround.
Connection timeout, refusal, or “network unreachable”
These messages indicate a path problem before SMTP authentication or message content is evaluated. First resolve the host name and identify the destination address; then test whether the VPS can open the required port.
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getent hosts smtp.example.net
nc -vz smtp.example.net 25
nc -vz smtp.example.net 587The exact diagnostic utility may not be installed, and a successful TCP test does not guarantee that the SMTP conversation or authentication will succeed. Check the VPS host firewall, cloud security rules, local egress policy, and the provider’s outbound filtering. The Postfix SMTP client records protocol and connection failures in the mail logs, so correlate every test with the queue ID.
Determine whether your VPS blocks outbound port 25
Server-to-server SMTP normally uses TCP port 25. Many VPS networks restrict or filter outbound port 25 to limit abuse. The restriction can be provider-wide, applied to a new account, or removed only after an abuse review; it is not necessarily a Postfix configuration error.
Ask the VPS provider whether outbound port 25 is blocked and test a destination that should accept SMTP. Test from the VPS itself, not from your laptop. A timeout or refusal on every external port-25 destination, while ordinary outbound traffic works, is strong evidence of an egress policy. A single remote refusal is not proof of a provider block because recipient servers can impose their own limits.
Use authenticated submission on port 587
Port 587 is the submission port reserved for mail user agents sending mail to a mail transfer agent, as described in Debian’s Postfix guidance and RFC 2476. It is the practical alternative when direct delivery on port 25 is unavailable. A typical Postfix relay declaration is:
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relayhost = [smtp.example.net]:587The square brackets prevent MX lookups for the relay host. Replace the example host with the exact name supplied by your relay service and follow that service’s requirements for username, password, SASL mechanism, and TLS.
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The following pattern shows the types of settings involved; exact values and mechanisms depend on the relay service:
relayhost = [smtp.example.net]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
smtp_tls_CApath = /etc/ssl/certsStore the relay credentials in the password map with permissions that prevent other users from reading them, generate the map using the Postfix-supported tool, and reload Postfix after changes. Do not paste a real password into a public configuration example. If the provider requires a different SASL method, certificate policy, or submission port, use its documented values instead of copying this example unchanged.
- Set the relay hostname and port in the Postfix configuration.
- Enable SASL only as required and place the relay username and password in the protected password map.
- Set TLS to the provider’s required level and ensure the CA trust store is present.
- Reload Postfix, submit one new test, and watch for a new queue-ID result in the log.
An authenticated relay changes the delivery path: your VPS submits to the relay, and the relay performs onward delivery. Compare its sending limits, logging, geographic processing, and cost with your need for direct control.
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Keep relay restrictions secure
Postfix’s relay controls are designed to stop unauthorized bulk mail. A safe policy distinguishes local or trusted clients from untrusted Internet hosts and requires authentication for remote users. Review these areas together:
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smtpd_relay_restrictions: retain explicit destination and authorization checks. Do not replace them with a blanket permit rule.- Listener exposure: expose the submission service only where needed, and apply firewall rules appropriate to your clients.
After changing authorization, test both a legitimate authenticated submission and an unauthorized attempt from outside your trusted network. The first should be accepted; the second should be rejected rather than relayed.
Validate DNS and sender identity after transport works
Getting a message to the relay or recipient server is only the transport step. Remote acceptance and inbox placement also depend on the identity attached to the message. Verify all of the following for the sending domain and IP:
- Forward DNS: the sending hostname resolves to the VPS address used for SMTP.
- Reverse DNS: the VPS provider’s PTR record points the sending IP to a stable hostname, and that hostname resolves forward to the same address.
- SPF: publish an SPF record authorizing the VPS IP or the relay service that actually sends the message.
- DKIM: sign outgoing messages and publish the corresponding public key in DNS when your delivery path supports it.
- DMARC: publish a policy and ensure the visible From domain aligns with the SPF or DKIM authenticated domain.
These records address remote rejection and spam handling; they cannot start a stopped Postfix service, open a blocked port, or correct a missing relay password. If a recipient accepts the SMTP transaction but the message is absent from the inbox, inspect the recipient’s spam, quarantine, and authentication-result details rather than changing queue settings blindly.
Choose direct delivery or an authenticated relay
Once Postfix itself is healthy, choose the delivery path that fits the VPS and your operational requirements.
| Consideration | Direct-to-recipient delivery | Authenticated relay on port 587 |
|---|---|---|
| Network requirement | Outbound port 25 must be permitted, and recipient servers must be reachable. | Requires access to the relay’s submission port, commonly 587. |
| Authentication | Your server presents its own SMTP identity to each recipient. | Your VPS authenticates with the relay using its required credentials and SASL method. |
| TLS | Negotiated separately with each recipient server; failures appear in your Postfix logs. | Configured for the relay’s hostname, certificate chain, and security policy. |
| DNS control | You need reliable forward and reverse DNS for the VPS and sender-domain authentication. | The relay may provide or control the onward-sending IP identity; your domain still needs appropriate SPF, DKIM, and DMARC alignment. |
| Diagnostics | You see direct connection and recipient responses in your own logs. | You see submission results locally, while onward-delivery details depend on the relay’s logs and reporting. |
| Limits and cost | Limited by your VPS reputation, provider policy, recipient throttling, and infrastructure. | Subject to the relay’s plan, rate limits, authentication rules, and any recurring charge. |
Direct delivery is reasonable when the provider allows port 25 and you can maintain DNS, reputation, TLS, and monitoring. An authenticated relay is usually simpler when port 25 is blocked or when you need a managed outbound reputation and submission service.
A complete verification procedure
- Confirm
systemctl status postfixreports a running service. - Start following the distribution’s mail log or journal.
- Send one message to a mailbox you own with a unique subject.
- Record its queue ID and inspect
postqueue -pormailq. - Classify the result as delivered, deferred, bounced, or still active.
- For a deferral or bounce, preserve the complete SMTP response and correct that specific cause.
- If using a relay, verify the relay hostname, port 587 access, SASL credentials, and TLS verification.
- Check forward and reverse DNS plus SPF, DKIM, and DMARC alignment.
- Repeat the controlled test and follow the new queue ID to a definitive result.
A successful test proves delivery only for that destination and configuration at that time. It does not guarantee that every recipient will accept the message or place it in the primary inbox. Keep the queue ID, timestamp, destination, and final SMTP response with your operational notes so the next failure can be compared against a known-good transaction.
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