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When Sendmail is “running” but mail never arrives, start with evidence rather than restarting it repeatedly. A running process does not prove that Sendmail is listening on the required interface, accepting relay requests, or able to reach a recipient’s mail server. Check the listening socket, read the mail log while reproducing one test, and then separate local submission problems from remote-delivery problems.
The sequence below covers the common failure points on a Linux VPS: service and listener state, queue and log evidence, hostname and DNS identity, relay policy, outbound TCP port 25 access, and STARTTLS. It also shows when direct delivery should be replaced with an authenticated SMTP relay.
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Sendmail has several independent stages. A message can be accepted by the local command, placed in the queue, handed to a remote host, deferred for retry, or rejected permanently. Seeing one successful stage does not prove that the next one succeeded.
- Local submission: a process or application hands a message to Sendmail.
- Queueing: Sendmail records the message for immediate or deferred delivery.
- SMTP connection: the VPS resolves the recipient domain and connects to its mail server.
- Remote policy: the receiving server accepts the sender, recipient, and message.
- Encryption and authentication: STARTTLS and SMTP credentials work when the selected route requires them.
A local enqueue proves only that the local mail system accepted the message. It does not prove remote delivery. Treat each stage as a separate test.
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1. Confirm the installed package, service, and version
First identify which Sendmail package is installed and which service unit your distribution uses. The unit may be named differently across distributions, so do not assume that a single service name applies everywhere.
- Use the distribution’s package manager to confirm that Sendmail and its configuration package are installed, and record the installed version.
- Check the service status with the unit name reported by your distribution, for example
systemctl status sendmail. On systems using a legacy service wrapper, the equivalent may beservice sendmail status. - Review recent service messages with the matching journal command, such as
journalctl -u sendmail, or the platform’s service-log viewer. - After any configuration change, reload or restart the same unit and confirm that it remains active instead of assuming the command succeeded.
Startup errors involving configuration syntax, permissions, aliases, queue directories, or hostname qualification belong to this stage. Fix them before testing remote delivery.
2. Verify the SMTP listening socket
One documented failure mode is a Sendmail daemon that is running but is not listening on TCP port 25, or is listening on port 587 instead. Check the socket directly:
sudo ss -ltnp | grep -E ':(25|465|587)b'Interpret the result by address as well as port:
0.0.0.0:25or[::]:25indicates a listener on all IPv4 or IPv6 interfaces, subject to firewall rules.127.0.0.1:25accepts connections only from the VPS itself. A remote application or external SMTP client cannot connect to it.- A listener on 587 is a submission endpoint, not proof that direct-to-recipient delivery on port 25 is available.
- No matching line means the daemon is not listening on any of the checked ports, even if its process appears active.
Compare the actual socket with /etc/mail/sendmail.mc and the generated sendmail.cf, paying particular attention to DAEMON_OPTIONS. A configuration that binds only to loopback may be intentional for local applications; do not expose a public listener merely to make a test pass.
3. Read the mail log while reproducing one controlled test
Sendmail writes through syslog’s LOG_MAIL facility. Depending on the distribution, the relevant records are commonly in /var/log/maillog or /var/log/syslog; some systems also provide /var/log/mail.log. Use the file that exists on the VPS:
sudo tail -F /var/log/maillog /var/log/mail.log /var/log/syslogIf several paths exist, watching all of them is useful during one test, but do not mistake an unchanged file for proof that Sendmail produced no event. Confirm which file receives LOG_MAIL entries on the system.
Submit one test message while the log is visible, then follow its queue and delivery records. Important fields include:
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relay=: the host that accepted or attempted the SMTP handoff.dsn=: the SMTP delivery-status classification.stat=orstatus=: the human-readable result, such as sent, deferred, or a permanent failure.- Connection timeouts, DNS lookup failures, authentication errors, certificate errors, and explicit 550 relay responses.
Sendmail records receipt and delivery attempts separately. A message can therefore appear as accepted locally and later show several delivery attempts when the first attempt is deferred. Use the message identifier in the log to follow the complete path rather than reading only the first line.
4. Separate local submission from remote delivery
Classify the failure before changing configuration.
Local submission fails immediately
Concentrate on the local daemon, permissions, aliases, configuration syntax, and queue directories. The application may be unable to connect to the local socket, or Sendmail may reject the envelope before creating a queue entry. The service status, journal, and mail log should show the exact rejection.
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Local acceptance is working. Investigate recipient-domain DNS, outbound connectivity, port 25 policy, TLS, and the remote server’s response. A deferred queue entry is not a delivered message; it means Sendmail intends to retry according to its queue policy.
The log reports a permanent failure
Read the SMTP response and dsn together. A permanent recipient rejection, a sender-policy failure, or a relay denial requires a policy or identity correction, not repeated restarts.
5. Use Sendmail’s built-in diagnostics before sending more mail
Two modes test configuration without creating a normal delivery attempt:
sendmail -bv user@example.com
sendmail -btsendmail -bv: verify a name
-bv verifies how Sendmail resolves a recipient name without collecting or delivering a message. Use it to detect invalid aliases, unresolved local names, and address-qualification problems while keeping the test non-delivery.
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sendmail -bt: test rewriting and rules
-bt enters address-test mode. It exercises the rewriting rules and configuration tables used to transform addresses. The Sendmail debugging guidance specifically uses this mode to inspect check_rcpt and check_relay behavior when diagnosing relay decisions.
Run these tests after recording the active configuration. If a rule change appears to have no effect, verify that the generated configuration was rebuilt and that the running daemon loaded it.
6. Correct the VPS hostname and DNS identity
Sendmail needs a stable, fully qualified local hostname for startup and for the identity it presents during SMTP. Verify all of the following before editing rewrite rules:
- The VPS’s effective hostname is a fully qualified name rather than a transient provider label.
- Forward DNS for that name resolves to the VPS’s intended public address.
- Reverse DNS (PTR) is configured where the provider permits it and points back to the same identity or an appropriate mail hostname.
- The resolver configured on the VPS can resolve recipient domains and their MX records.
Inability to qualify the local hostname can delay startup or produce malformed outbound identity. A syntactically valid message can still be rejected by remote policy when the hostname, forward DNS, and reverse DNS do not agree. Correct the VPS and DNS records first; only then adjust address-rewriting rules.
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7. Fix “550 relaying denied” without opening an abuse hole
Sendmail 8.9 and later deny relaying by default. The Sendmail documentation states: “Relaying is denied by default.” This protects a new installation from becoming an open relay, but it also means that a legitimate application or trusted host must be identified explicitly.
Read the exact rejection
Use the log line to identify the connecting IP address, the envelope sender, and the recipient domain or class. A 550-style response may mean that the client is not trusted, the recipient is outside the server’s local domains, or an access-map rule is denying the request.
Check client identity and relay classes
Inspect the effective configuration for local domains, trusted networks, and relay classes. Then use sendmail -bt to test check_rcpt and check_relay decisions for the same sender, client address, and recipient that produced the error.
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Review the access database
If the installation uses /etc/mail/access, make the narrowest change that matches the real requirement. Permit only the specific host, user, or domain that must relay. Do not respond to a 550 error by enabling unrestricted relay for all networks.
After editing the text database, rebuild its database with the distribution’s documented makemap command, then reload or restart Sendmail. Test again and confirm in the log that the intended client is accepted while an unrelated client remains refused.
8. Determine whether the VPS or network blocks outbound port 25
Direct delivery requires a connection from the VPS to the recipient domain’s MX host on TCP port 25, Sendmail’s default SMTP port. A timeout or refusal at that point indicates a firewall, VPS-provider policy, or upstream-network problem; it is not necessarily a Sendmail syntax error.
- Resolve the recipient domain’s MX records from the VPS.
- Select one returned MX hostname and test TCP connectivity to port 25 using the network diagnostic available on the distribution.
- Compare the result with the Sendmail log. A connection timeout, refusal, or unreachable route before an SMTP banner indicates a network-path issue.
- Check both the VPS firewall’s egress rules and the provider’s documented outbound-mail policy. Some providers block port 25 on new or low-trust instances.
Do not infer that port 25 is open because local submission works or because a listener exists. Inbound listening and outbound connectivity are separate controls.
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9. Choose direct delivery or an authenticated SMTP relay
If the provider blocks direct port 25, or if the VPS lacks the DNS and reputation needed for dependable delivery, configure an authenticated submission relay on a provider-supported port. That is an architecture change: Sendmail will hand mail to the relay, and the relay will deliver it to the recipient.
| Consideration | Direct-to-recipient delivery | Authenticated SMTP relay |
|---|---|---|
| Outbound port | Requires reachable TCP port 25 from the VPS to recipient MX hosts. | Uses the relay provider’s supported submission port; direct recipient port 25 access is not required. |
| Authentication | Recipient servers evaluate the VPS and domain identity directly. | Sendmail authenticates to the relay with credentials and follows that service’s sender requirements. |
| TLS | Negotiated independently with each recipient server when offered or required. | Configured for the single relay endpoint, which may require STARTTLS. |
| DNS and reverse DNS | Stable hostname, forward DNS, and applicable reverse DNS are important to acceptance. | The relay’s infrastructure handles recipient-facing delivery, but your envelope and domain policies still must meet its rules. |
| Queue and retries | Sendmail queues and retries against each recipient MX. | Sendmail queues until the relay accepts the message; the relay then manages onward delivery and its own retries. |
| Observability | Logs show each recipient MX connection and response. | Local logs show the relay handoff; downstream status is reported by the relay and its logs or interface. |
| Provider policy | Subject to VPS egress restrictions and recipient reputation checks. | Subject to the relay account’s authentication, TLS, sender, quota, and abuse policies. |
| Operational cost | No relay-service charge is implied, but DNS, reputation, monitoring, and abuse handling remain yours. | Any service fee, quota, and retention policy depends on the selected relay and must be verified separately. |
Choose direct delivery only when the VPS network permits it and your domain identity can meet recipient policy. Choose an authenticated relay when port 25 is blocked or direct delivery is consistently unreliable. Do not treat a relay as a bypass for forged sender addresses; use credentials and sender identities authorized by that service.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.10. Diagnose STARTTLS independently
TLS failures are distinct from listener and relay-policy failures. A successful TCP connection proves only that a socket is reachable. It does not prove that Sendmail and the peer can negotiate STARTTLS or authenticate.
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- Connect to the local SMTP listener or the configured relay endpoint with an SMTP-capable diagnostic tool.
- Inspect the server’s advertised capabilities and confirm whether
STARTTLSappears. - Attempt the TLS negotiation and record certificate, protocol, and authentication errors.
- Review the Sendmail log at the same time for certificate-file permissions, unsafe-file warnings, failed handshakes, or authentication responses.
Check that private keys and certificates are readable by the Sendmail process but not writable by untrusted users. Correct file ownership and permissions, then reload the daemon. Test TLS again before changing relay access rules; solving a 550 policy error will not repair a certificate or handshake failure.
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No listener on port 25
Check DAEMON_OPTIONS, generated configuration, and the service journal. Rebuild the configuration if the source file was edited, restart Sendmail, and repeat the ss check. If the daemon binds only to loopback by design, use a local submission path or a deliberately configured submission listener rather than exposing port 25.
Listener exists, but local submission is rejected
Inspect permissions, aliases, queue directories, and the exact log response. Run sendmail -bv for the recipient and sendmail -bt for address rewriting. Correct local configuration before testing DNS or provider policy.
Message is queued and repeatedly deferred
Read the latest stat, dsn, and relay values. Resolve the recipient MX, test outbound port 25, and check for DNS or timeout errors. If the network path is blocked, move to an authenticated relay instead of shortening retry intervals.
Remote host says relay denied
Confirm whether the rejection concerns your VPS relaying to an outside domain or the recipient’s own policy. For a local Sendmail 550, inspect client identity, local domains, relay classes, and the access database. For a recipient-side rejection, use the remote SMTP response and your hostname, forward DNS, reverse DNS, and sender policy as the next evidence.
STARTTLS or authentication fails
Verify advertised capabilities, certificate and key permissions, protocol negotiation, and credentials. Keep this investigation separate from port-listener and relay-access changes so that each test has one clear variable.
12. A repeatable verification checklist
After making a change, verify the complete path in this order:
- The correct Sendmail package and service unit are installed, active, and free of recent startup errors.
- The expected listener appears in
ss, on the intended address and port. - The VPS hostname is fully qualified, and forward and applicable reverse DNS are consistent.
sendmail -bvresolves the test recipient without delivery.sendmail -btproduces the expected rewriting,check_rcpt, andcheck_relaydecisions.- A controlled message creates a local receipt record in the mail log.
- The queue or delivery record shows the expected
relay,dsn, andstatvalues. - The VPS can reach the intended recipient MX on port 25, or the configured relay endpoint on its supported port.
- STARTTLS and authentication succeed when the route requires them.
- The final log response is a successful remote acceptance, not merely local enqueue.
Save the relevant log lines for the test message. They provide the evidence needed to distinguish a Sendmail configuration fault from a DNS, firewall, provider, TLS, or recipient-policy fault.
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