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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Configure Ubuntu’s Postfix service as a restricted smarthost, and it will hand outgoing messages to Mailgun instead of connecting directly to recipient mail servers. The practical default is Mailgun’s [smtp.mailgun.org]:587 endpoint with SASL authentication and STARTTLS. Store the sending-domain SMTP credential in a root-only password map, require authenticated or trusted local senders, and verify the result with a real message, the Postfix queue, and the mail logs.
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This arrangement gives local applications a standard SMTP service while Mailgun handles the remote SMTP delivery. It does not make the Ubuntu server an open relay: destinations that are not authorized by your local policy must be rejected.
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What the relay changes
Without a relay host, Postfix attempts to deliver remote mail directly to each recipient domain’s MX server. With relayhost configured, Postfix sends every remote message to the specified SMTP server instead. Mailgun then accepts the authenticated submission and performs the onward delivery.
- Applications submit mail to Postfix on the Ubuntu host.
- Postfix authenticates to Mailgun with the SMTP credentials belonging to the sending domain.
- Mailgun receives the message over the selected submission port and handles remote delivery.
- Postfix retains queue and status information locally, so failures remain visible in its logs.
The relay host setting is specifically designed for this smarthost pattern: it forces remote messages to the named server rather than having Postfix deliver to destination MX records itself.
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Prerequisites and decisions
- An Ubuntu server with
sudoaccess. - A resolvable hostname for the server. A stable hostname makes mail-service diagnostics and TLS troubleshooting easier.
- A Mailgun sending domain with SMTP credentials created for that domain.
- Outbound TCP access from the server to the Mailgun submission port you choose.
- A local application, command-line mailer, or test utility that can submit a message to Postfix.
Mailgun manages SMTP credentials per sending domain. The password for the Mailgun control panel is not the password that belongs in Postfix. Obtain the SMTP username and password shown for the specific sending domain instead.
Confirm the sending domain is configured and authorized in Mailgun before testing. Use a sender address at that domain, and choose a recipient you control so you can verify both the SMTP transaction and receipt. The Ubuntu hostname should resolve consistently in local DNS and, where applicable, public DNS; inconsistent names make log correlation and certificate troubleshooting harder even though Mailgun remains the outbound relay.
Install Postfix and select a smarthost configuration
Update Ubuntu’s package metadata and install Postfix:
sudo apt update
sudo apt install postfixDuring package configuration, select the smarthost-oriented option (usually labelled Internet with smarthost) when the installer asks how mail should be sent. This expresses the intended design: local programs can submit mail, while outbound delivery is handed to another SMTP server. If the package was already installed with a different choice, the files can still be corrected manually in the following steps.
Ubuntu’s Postfix package uses a default chroot arrangement for some service processes. Do not disable that arrangement merely to make Mailgun work; first apply the provider and TLS settings, then inspect logs if a process-specific problem appears.
Configure the Mailgun relay in main.cf
Edit the main Postfix configuration:
sudo editor /etc/postfix/main.cfSet the relay and SMTP authentication parameters as follows:
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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 = may
smtp_tls_note_starttls_offer = yesThe square brackets in [smtp.mailgun.org]:587 tell Postfix to use that host and port as the relay endpoint rather than looking up a destination MX record for the relay name. The key in the password map must match the relay host, including the brackets and port.
The noanonymous setting prevents Postfix from selecting an anonymous SASL mechanism. Authentication is enabled, and the password map supplies the account that Mailgun issued for the sending domain.
Keep one authoritative relayhost declaration. Duplicate settings later in main.cf can override earlier lines and make troubleshooting confusing. Use postconf after editing to see the value Postfix actually loaded rather than relying on what appears in an editor.
Create and protect the SASL password map
Create the plaintext source file with one line containing the exact relay key, followed by the SMTP username and password:
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sudo editor /etc/postfix/sasl_passwd[smtp.mailgun.org]:587 SMTP_USERNAME:SMTP_PASSWORDReplace SMTP_USERNAME and SMTP_PASSWORD with the credentials for the Mailgun sending domain. Do not use the control-panel password. The colon separates the username from the password; if a password itself contains characters that are meaningful to your editing or deployment process, enter it exactly as Mailgun supplies it and avoid exposing it in shell history.
Compile the map and restrict both files:
sudo postmap /etc/postfix/sasl_passwd
sudo chown root:root /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
sudo chmod 600 /etc/postfix/sasl_passwdPostfix reads the generated database file, while the original file remains a plaintext secret. Keep it readable only by root, remove accidental copies from shared directories and shell transcripts, and ensure backups do not make the credential world-readable. When the password changes, edit the source file, run postmap again, and reload or restart Postfix so the new map is used.
Do not put the password directly in main.cf, a unit-file command line, or a world-readable configuration-management template. Those locations commonly appear in support bundles, process listings, or backups. After a rotation, test a fresh message and remove obsolete map copies so an old credential cannot be recovered from an unprotected directory.
Keep the Ubuntu host from becoming an open relay
Authentication to Mailgun and authorization for clients connecting to your Ubuntu server are separate controls. Limit who may submit mail locally and reject destinations that are not authorized. A typical recipient restriction order is:
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemssmtpd_recipient_restrictions = permit_sasl_authenticated,permit_mynetworks,reject_unauth_destinationThe final reject_unauth_destination clause is essential. It prevents an unauthenticated client from using your Postfix instance to relay to arbitrary domains. The earlier clauses permit authenticated SMTP clients and hosts listed in Postfix’s trusted mynetworks setting.
Review mynetworks rather than expanding it casually. Include only loopback addresses and private application networks that you actually control. Do not add the whole internet, a broad public address range, or an untrusted cloud network just to make a test succeed. If external users must submit mail, require SMTP authentication and keep the unauthenticated-destination rejection in place.
Test this boundary deliberately after setup: submit one message from an approved local application, then try an unauthorized destination from an untrusted client or network. The second attempt should be rejected before Postfix queues it. A successful local submission alone does not prove relay protection.
Choose a Mailgun submission port and TLS policy
Mailgun documents four SMTP ports. Their encryption and reachability characteristics differ:
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →| Port | Connection mode | Operational consideration | Postfix implication |
|---|---|---|---|
| 25 | Plain SMTP initially, with STARTTLS available | Frequently blocked or throttled by hosting providers | Use only when outbound access is dependable and policy allows it |
| 465 | Implicit TLS from the beginning of the connection | Requires a client configuration for immediate TLS | Not interchangeable with the STARTTLS settings shown for port 587 |
| 587 | Plain SMTP initially, upgraded with STARTTLS | Mailgun’s practical default when port 25 is restricted | Use [smtp.mailgun.org]:587 with the configuration above |
| 2525 | Plain SMTP initially, with STARTTLS available | An alternative when common SMTP ports are filtered | Change both relayhost and the password-map key to port 2525 |
Port 587 is generally the best starting point because it supports authenticated submission with STARTTLS and is less likely than port 25 to be blocked. If you select 2525 instead, use a matching key such as [smtp.mailgun.org]:2525 in both main.cf and sasl_passwd. A mismatch causes authentication lookup failures even when the username and password are correct.
Port 465 is not a drop-in substitution for this configuration. It starts with implicit TLS, whereas the sample expects a plaintext greeting followed by STARTTLS. Select it only when your Postfix version and chosen SMTP client settings explicitly support that mode. Ports 25 and 2525 use the same STARTTLS style as 587, but their network reachability and provider policies may differ.
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The sample uses smtp_tls_security_level = may. With that policy, Postfix attempts to upgrade the connection when the server offers STARTTLS, but delivery can fall back if TLS negotiation is unavailable. For a stricter deployment, change the setting to:
smtp_tls_security_level = encryptRequired encryption changes the failure behavior: if TLS cannot be established, Postfix does not deliver the message. Test certificate validation, firewall reachability, and the remote endpoint before enforcing this setting in production. Optional TLS is more tolerant of a broken upgrade path, but it can permit plaintext fallback, so the choice is a security-policy decision rather than a cosmetic preference.
Ensure the Ubuntu CA bundle is installed and that Postfix can locate it. A common Ubuntu CA file is:
/etc/ssl/certs/ca-certificates.crtIf certificate errors occur, check that the CA package is present, the server clock is accurate, and the hostname used for the SMTP connection matches the certificate validation path. Do not “fix” a certificate failure by disabling validation or switching to an unauthenticated relay.
Check the resulting configuration before restarting
After saving main.cf and the password map, ask Postfix to validate its files:
sudo postfix checkA successful check produces no error output. Confirm the effective values for the settings that matter:
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sudo postconf relayhost smtp_sasl_auth_enable smtp_sasl_password_maps smtp_sasl_security_options smtp_tls_security_level smtp_tls_note_starttls_offer smtpd_recipient_restrictionsRestart the service so the new configuration and map are loaded:
sudo systemctl restart postfix
sudo systemctl --no-pager --full status postfixThe status output should show an active Postfix service. If it fails, run postfix check again and read the service journal before attempting another restart; syntax errors, ownership problems, and malformed map entries are usually reported there.
Send a test message through the local Postfix path
A useful test exercises the same path that your application will use: submit to the local Postfix service, then let Postfix authenticate to Mailgun. For a simple command-line test, provide a sender, recipient, subject, and body through sendmail:
printf 'From: [email protected]: [email protected]: Postfix Mailgun relay testnnThis is a relay test.n' | sendmail -v [email protected]Use a sender address belonging to the sending domain configured in Mailgun and a recipient address you control. The verbose output shows whether the local submission was accepted; remote acceptance is confirmed in the Postfix log and in Mailgun’s message records, not solely by the local command returning.
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For a direct SMTP diagnostic, Mailgun documents a Swaks pattern that supplies the server, SMTP username, SMTP password, recipient, subject, and body. Keep the password out of shell history when possible by entering it through an environment variable or a protected prompt:
read -r SMTP_USERNAME
read -r -s SMTP_PASSWORD
swaks --server smtp.mailgun.org:587
--auth LOGIN
--auth-user "$SMTP_USERNAME"
--auth-password "$SMTP_PASSWORD"
--from [email protected]
--to [email protected]
--header 'Subject: Direct Mailgun SMTP test'
--body 'Testing Mailgun SMTP authentication.'The direct test isolates Mailgun credentials and network access. The local sendmail test additionally proves that your application-to-Postfix path works. Run both only when you need to distinguish those two layers.
Verify the queue and logs
Immediately after submitting the test, inspect the queue and recent service messages:
mailq
sudo journalctl -u postfix -n 100 --no-pager
sudo tail -n 100 /var/log/mail.logA successful relay normally shows SASL authentication to the Mailgun endpoint followed by acceptance of the remote message. The queue should empty after delivery. A message that remains queued with repeated deferrals has not been delivered yet; read the exact SMTP response before changing settings.
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Correlate the queue ID across the journal, /var/log/mail.log, and Mailgun’s event or message records. A local “queued” response means only that Postfix accepted responsibility. It does not prove Mailgun accepted the message or that the recipient mailbox delivered it. Capture the timestamp, destination, response code, and queue ID when escalating an issue.
Troubleshoot the common failure modes
Authentication failed
Verify that the credential is the SMTP username and password for the correct Mailgun sending domain, not the control-panel password. Then compare the relay key character-for-character in both files. For port 587, both must use [smtp.mailgun.org]:587. After correcting the source file, rerun postmap /etc/postfix/sasl_passwd and restart or reload Postfix.
TLS or certificate errors
Check outbound connectivity to the selected port, the Ubuntu CA bundle, the server clock, and the hostname in relayhost. If you set smtp_tls_security_level = encrypt, any failure to negotiate TLS will defer delivery by design. Temporarily evaluating the endpoint with the documented may policy can help distinguish a strict-policy failure, but do not leave a security exception in place without deciding whether plaintext fallback is acceptable.
Messages stay queued or are repeatedly deferred
Read the SMTP response in /var/log/mail.log and the system journal. Check DNS resolution, outbound firewall rules, the selected port, and Mailgun account or sending limits. A blocked port 25 is a common reachability issue; try the supported submission port 587 or, where appropriate, 2525. Do not infer success from local queue acceptance alone.
The local application cannot submit
Confirm that Postfix is active, that the application is connecting to the local SMTP service, and that its source address is covered by the intended mynetworks policy or that it authenticates. Keep reject_unauth_destination enabled while making this adjustment. Broadening trusted networks is not a safe substitute for diagnosing the application’s actual connection address.
A configuration change appears to have no effect
Run postfix check, inspect effective values with postconf, regenerate the password database, and restart Postfix. Editing sasl_passwd without running postmap leaves the database unchanged, so Postfix continues using the old credential.
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Port connection times out
Test DNS resolution and the outbound firewall separately from authentication. A timeout usually indicates filtering or routing, not a bad password. If port 25 is blocked or throttled, use 587; if that is filtered, Mailgun documents 2525 as another STARTTLS-capable option. Keep the relay and password-map port numbers synchronized whenever you switch.
Operate the relay safely after it works
- Rotate the Mailgun SMTP password when required, update the root-only source file, regenerate the map, and restart Postfix.
- Review file ownership and mode after package upgrades, configuration-management runs, and restores from backup.
- Monitor the queue and mail logs for authentication failures, repeated deferrals, and unexpected volume.
- Keep outbound firewall rules limited to the submission ports you actually use.
- Retain the destination rejection rule and periodically verify that an unauthorized client cannot relay to an arbitrary domain.
- Document the selected port, TLS policy, credential owner, and the local applications permitted to submit mail.
Plan credential rotation and incident response before the first production message. If a password is exposed, revoke or replace it in Mailgun immediately, update the protected map, regenerate the database, and review logs for unexpected submissions. Restrict administrative access to the Ubuntu host because anyone who can read the map can impersonate the sending domain through your relay.
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| Check | Expected result |
|---|---|
| Relay endpoint | [smtp.mailgun.org]:587, or a deliberately selected supported port |
| Credentials | SMTP credentials for the Mailgun sending domain, not the control-panel password |
| SASL | smtp_sasl_auth_enable = yes and smtp_sasl_security_options = noanonymous |
| Password map | hash:/etc/postfix/sasl_passwd, regenerated with postmap, source mode 600 |
| TLS | STARTTLS offered on 587, CA validation available, and policy chosen intentionally |
| Relay protection | permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination |
| Validation | postfix check succeeds and the service restarts cleanly |
| Proof of delivery | A real test message is accepted, the queue clears, and logs show authenticated remote delivery |
Frequently Asked Questions
Which Mailgun SMTP port should I use with Postfix?
Start with port 587 and the relay key [smtp.mailgun.org]:587. It uses STARTTLS and is generally more reachable than port 25. Port 2525 is another STARTTLS option when common SMTP ports are filtered. Port 465 uses implicit TLS and requires a different client mode, so it is not interchangeable with the sample configuration.
Can I use my Mailgun dashboard password?
No. Mailgun issues SMTP credentials for each sending domain. Use that domain’s SMTP username and password in /etc/postfix/sasl_passwd; the control-panel password is not the relay credential.
Why does Postfix say no SASL password was found?
The password-map key normally does not match relayhost exactly. Include the brackets and port, such as [smtp.mailgun.org]:587, rerun postmap /etc/postfix/sasl_passwd, and reload or restart Postfix.
Should TLS be set to may or encrypt?
may permits an upgrade when STARTTLS is offered and can fall back if negotiation fails. encrypt requires TLS and defers delivery whenever encryption cannot be established. Choose encrypt only after confirming reachability, CA validation, hostname matching, and correct server time.
How do I know the server is not an open relay?
Keep reject_unauth_destination after permit_sasl_authenticated,permit_mynetworks, restrict mynetworks to controlled addresses, and test from an unauthorized client. The unauthorized attempt should be rejected rather than queued for an arbitrary destination.
Does a successful sendmail command prove delivery?
It proves that Postfix accepted the local submission. Confirm remote acceptance by finding SASL authentication and a successful Mailgun response in the logs, seeing the queue clear, and checking the recipient mailbox or Mailgun event records.
What should I do after changing the SMTP password?
Edit the root-only source map, run postmap /etc/postfix/sasl_passwd, then reload or restart Postfix. Verify ownership and mode again and send a new test message; editing only the plaintext file leaves the old database entry active.
Why are messages repeatedly deferred?
Read the exact SMTP response in /var/log/mail.log and the system journal. Check DNS, outbound firewall rules, the selected port, TLS certificate validation, Mailgun account limits, and the queue ID. A local queue acceptance is not proof that Mailgun accepted the message.
The Bottom Line
Use Postfix as a restricted smarthost: configure [smtp.mailgun.org]:587, authenticate with the sending domain’s SMTP credentials, protect and regenerate the SASL map, and keep reject_unauth_destination enabled. Choose TLS policy deliberately, then prove the path with a real message, an empty queue, and authenticated delivery entries in the logs.
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