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When Postfix is “not sending” mail, begin with the queue ID and the exact SMTP or connection error—not the local send command alone. A message may be accepted by Postfix, assigned a queue ID, and then remain deferred because DNS, networking, TLS, a remote server, or your VPS provider prevented delivery.
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The reliable fix is to trace one test message from submission through the queue and logs, classify the failure, correct that specific layer, and then verify delivery to a mailbox you control. The runbook below covers service failures, stuck queues, relay authorization, blocked port 25, TLS errors, and sender-authentication problems.
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Start with a queue ID and the real error
“Postfix is not sending” describes an outcome, not a diagnosis. First establish whether the service is running, whether it accepted the message, and what happened afterward.
1. Confirm that Postfix is running
- Check the service:
systemctl status postfix - If it is stopped or failed, inspect the status output and recent service logs before restarting. A restart can clear a transient process problem, but it will not fix blocked port 25, bad DNS, invalid credentials, or a remote rejection.
- Submit one test message with a distinctive subject and a recipient mailbox you control. Send only one test at a time so its queue ID and log lines are easy to identify.
A successful local command means only that a local process handed the message to Postfix. It does not prove that the recipient server accepted it or that the message reached the inbox.
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2. Follow the mail logs while reproducing the problem
Ubuntu documents /var/log/mail.log as the main Postfix log and identifies /var/log/mail.err and /var/log/mail.warn for error and warning messages. Watch the log in one terminal while sending the test:
tail -f /var/log/mail.logOn distributions that route mail events to the system journal instead, use the equivalent journal or syslog view, such as:
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Inspect the queue before changing configuration
List queued mail with either command:
postqueue -p
mailqMatch the queue ID from your test to the log lines and classify the message. Postfix queue services retain status for failed, delayed, and successful deliveries.
| Queue result | What it means | What to do next |
|---|---|---|
| Active | The message is being processed for delivery. | Continue watching the log for the SMTP result; do not repeatedly submit duplicates. |
| Deferred | Delivery encountered a temporary problem, so Postfix retained the message for retry. | Read the reason after status=deferred; investigate DNS, connectivity, TLS, rate limiting, or the remote server. |
| Bounced | Postfix received a permanent failure and generated or attempted a bounce. | Use the SMTP response to correct the address, policy, authentication, or sender configuration before trying again. |
| Delivered | The destination server accepted the message. | If it is not visible, investigate recipient-side spam filtering, quarantine, or mailbox rules; acceptance is not the same as inbox placement. |
Do not delete a deferred message merely because it is old. The queue reason tells you whether retrying after a correction is appropriate. Once the underlying problem is fixed, you can request a queue retry with postqueue -f, then follow the same queue ID in the logs.
Fix the error shown in the log
status=deferred: temporary delivery failure
A deferred message normally stays queued while Postfix retries. The cause may be a destination server that is temporarily unavailable, DNS that cannot resolve the recipient domain, a network timeout, remote rate limiting, or a TLS negotiation problem.
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- Resolve the recipient domain and its MX records from the VPS. If DNS lookups fail or return inconsistent results, correct the VPS resolver configuration or the domain’s DNS before retrying.
- Check whether the VPS can open an SMTP connection to the destination. A timeout or unreachable network points to host firewall rules, a provider egress policy, routing, or a blocked port.
- Read the complete remote response. A temporary 4xx response calls for retry and possibly slower sending; a 5xx response is a permanent policy or address problem that needs a configuration or recipient correction.
- For TLS-related deferrals, verify the relay hostname, certificate name, trust chain, and configured security level.
Do not treat every deferral as a Postfix queue defect. The queue is doing its job by preserving the message instead of silently discarding it.
relay access denied: authorization is missing
This response means the SMTP client is not authorized to use the server as a relay. Postfix deliberately permits relaying only for configured trusted networks, authenticated clients, or permitted destinations. Review the effective values of mynetworks, SASL authentication settings, and smtpd_relay_restrictions.
A safe authorization model commonly allows:
permit_mynetworksfor explicitly trusted local networks;permit_sasl_authenticatedfor clients that successfully authenticate; and- a destination restriction that prevents unauthenticated users from relaying arbitrary domains.
Never solve a relay error by permitting every network or destination. An unrestricted SMTP server becomes an open relay, allowing abuse, blacklist listings, and resource exhaustion. After changing restrictions, reload Postfix and send a new test from an authorized client.
TLS certificate, hostname, or trust errors
When Postfix reports certificate verification or hostname errors, compare the hostname in relayhost with the name covered by the relay’s certificate. Check that the VPS trusts the issuing certificate chain and that the configured TLS security level matches the relay’s requirements.
A bracketed relay host such as [smtp.example.net]:587 tells Postfix to use that host and port rather than selecting a destination MX record. Use the exact hostname supplied by your relay service; do not substitute an unrelated IP address when certificate name checking is required.
Connection timeout, network unreachable, or connection refused
These errors occur before a complete SMTP transaction. Check, in order:
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- DNS resolution for the relay or recipient domain.
- Outbound firewall rules on the VPS and any cloud security-group or network policy.
- The VPS provider’s policy for outbound SMTP, especially port 25.
- The remote server’s availability and advertised port.
Postfix records protocol and connection failures in the mail logs. A timeout is not fixed by changing sender headers, SPF, or DKIM; those identity controls matter after a network path exists.
Determine whether port 25 is blocked
Server-to-server SMTP normally uses TCP port 25. Many VPS providers filter or block outbound port 25 to limit spam. The block may be absolute, rate-limited, or applied only to newly created accounts, so a local Postfix service can appear healthy while every external delivery attempt times out.
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The practical alternative is authenticated submission through a relay on port 587. Debian describes port 587, covered by RFC 2476, as the submission port reserved for mail user agents sending mail to a mail transfer agent. The relay accepts an authenticated submission and performs onward delivery, so your VPS does not need unrestricted direct-to-recipient access on port 25.
Configure an authenticated relay on port 587
Use the relay provider’s documented hostname, username, password mechanism, and TLS requirements. A typical Postfix pattern is:
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smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encryptThe exact directives and TLS mode must match the relay’s instructions. Some services require a specific authentication mechanism, an application password, or a different security level.
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Store credentials separately and protect them
- Create the password map file at the path configured in
smtp_sasl_password_maps. The key must match the relay host and port, followed by the supplied username and password. - Restrict the file so only root and the Postfix process can read it. Credentials in a world-readable file can expose the relay account.
- Build the database map with the appropriate
postmapcommand, then reload Postfix:
postmap /etc/postfix/sasl_passwd
systemctl reload postfixUse the file format and permissions recommended by your distribution. Never paste a real password into a public bug report or shell history.
Verify the relay configuration
Send one controlled message and watch the queue ID. A successful relay transaction should show connection to the configured host and a final delivery status. If the message defers, use the new SMTP response rather than assuming the relay is working. Authentication failures usually indicate an incorrect username, password, account policy, or required mechanism; TLS failures indicate a hostname, trust, or security-level mismatch.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Direct delivery versus an authenticated relay
Choose the delivery model that matches your VPS controls and operational requirements.
| Decision factor | Direct delivery to recipient MX servers | Authenticated relay on port 587 |
|---|---|---|
| Port requirement | Requires usable outbound port 25 and a reachable recipient SMTP server. | Uses the relay’s submission endpoint, commonly port 587, when port 25 is blocked. |
| Authorization | Your server must be accepted by each destination’s policy. | Your VPS authenticates once to the relay with the provider’s credentials. |
| TLS | Negotiated separately with each destination; failures appear in the Postfix log. | Configured against one relay hostname and its certificate requirements. |
| DNS and IP reputation | You control the sending IP, reverse DNS, forward DNS, SPF, DKIM, and DMARC, but must maintain them. | The relay supplies onward delivery infrastructure; your domain still needs aligned authentication records where required. |
| Diagnostics | Logs expose each destination’s connection and SMTP response. | Logs show the submission transaction; final recipient handling is partly inside the relay. |
| Cost and limits | Not stated; depends on VPS resources, provider policy, and destination acceptance. | Not stated; depends on the relay’s plan, sending limits, and credential policy. |
Direct delivery offers more control over the SMTP path but demands reliable port 25 access and a well-configured mail identity. A relay reduces network and reputation work at the VPS, but introduces credentials, provider limits, and dependence on the relay’s status.
Check DNS and sender identity after transport works
Transport and identity are separate layers. Fixing a stopped service or blocked port gets a message to an SMTP server; it does not guarantee acceptance or inbox placement.
Forward and reverse DNS
The sending hostname should resolve forward to the VPS’s sending IP, and the IP’s reverse-DNS (PTR) name should be correct and consistent with that hostname. Many providers control PTR records through the VPS dashboard or support process. A mismatch can trigger rejection or lower trust even when SMTP connectivity succeeds.
SPF
Publish an SPF record that authorizes the systems actually sending mail for your domain. If you use a relay, include the relay’s required authorization rather than authorizing only the VPS IP. Keep the record aligned with the envelope sender used by your application.
DKIM
DKIM signs messages so receiving systems can verify that authorized infrastructure sent them and that signed headers were not altered. Configure signing on the component that handles outbound mail—your Postfix stack or the relay, depending on the service—and publish the corresponding public key in DNS.
DMARC alignment
DMARC evaluates whether SPF and/or DKIM aligns with the visible From domain. Publish a policy appropriate to your deployment and monitor reports before tightening enforcement. A message can pass SMTP delivery yet be quarantined or rejected later if alignment fails.
These records address remote acceptance and spam handling. They cannot start a stopped Postfix service, unblock port 25, or repair an invalid relay password.
Common mistakes that keep the queue stuck
- Testing only the local command: A command that exits successfully proves local submission, not remote delivery. Always follow the queue ID to a final status.
- Reading only the application log: The decisive SMTP response is usually in the Postfix mail log or system journal.
- Flushing repeatedly before fixing the cause:
postqueue -fretries the same failure and can create noise or duplicate attempts. Correct the cause first. - Using an IP address for a TLS relay: Certificate verification commonly expects the relay hostname.
- Allowing all networks to relay: Broadening
mynetworksor relay restrictions can create an open relay. - Assuming port 587 is a universal replacement without credentials: Submission normally requires authentication and the relay’s specified TLS settings.
- Changing SPF to fix a timeout: DNS authentication cannot repair a blocked network path.
- Ignoring provider limits: A relay or VPS may impose sending limits or temporary rate controls; the SMTP response identifies whether that is happening.
- Deleting queued mail prematurely: Removal destroys the message and its diagnostic context. Preserve the queue ID and error until the cause is understood.
Verify the repair without overclaiming
- Send one message to a mailbox you own, using a unique subject and timestamp.
- Record the queue ID from the Postfix log.
- Watch the queue and logs until the message is delivered or a definitive bounce is recorded.
- Save the exact SMTP response, destination domain, and configuration change.
- Check the recipient’s spam or quarantine folder and inspect authentication results if the destination accepted the message but did not place it in the inbox.
This test proves the tested destination and configuration at that time. It does not guarantee delivery to every provider or universal inbox placement. If the result is still deferred, return to the exact error and work at the layer it identifies: queue and retry behavior, authorization, network path, TLS, or sender identity.
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Find the queue ID, read the exact Postfix log response, and fix that layer. Use an authenticated relay on port 587 when your VPS blocks outbound port 25; keep relay restrictions locked down, then validate DNS, SPF, DKIM, and DMARC for reliable acceptance and inbox placement.
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