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This guide covers the Docker-based self-hosted installation, including port 465 implicit TLS, alternate port 2525, sender-address restrictions, applying environment changes and diagnosing failed mail.
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What SMTP does in Discourse
Discourse uses SMTP (Simple Mail Transfer Protocol) to validate new accounts, send notifications, deliver password-reset messages and distribute other forum mail. The Discourse environment template describes the SMTP address as “The SMTP mail server used to validate new accounts and send notifications.” Your forum is an SMTP client: it connects to a relay operated by your mail provider, authenticates if required, submits a message and receives either an acceptance or a rejection.
This article assumes a conventional self-hosted Discourse Docker deployment under /var/discourse. A hosted Discourse service has a different administration model; you would not edit app.yml on that service.
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Before you edit app.yml
Collect these values from the current documentation for your SMTP provider:
- The SMTP hostname, such as
smtp.example.com. - The submission port and encryption mode. Port 587 with STARTTLS is common, but it is not universal.
- The SMTP username and password, if the service requires authentication.
- Any required SMTP domain, sender-domain verification or provider-specific TLS verification setting.
- A permitted From address. Some services require it to be the same mailbox used to authenticate.
You also need shell access with permission to modify files and operate the Discourse launcher. Back up /var/discourse/containers/app.yml before changing it, and treat the SMTP password as a secret: restrict file access and do not paste it into public support requests.
SMTP environment variables Discourse reads
Add the baseline variables under the existing env: mapping. Keep the indentation used by your generated container file; YAML indentation is significant.
| Variable | Purpose | What to enter |
|---|---|---|
DISCOURSE_SMTP_ADDRESS |
SMTP server hostname | The exact hostname supplied by your relay; this is required. |
DISCOURSE_SMTP_PORT |
TCP submission port | Usually 587 for authenticated STARTTLS, unless your provider specifies another port. |
DISCOURSE_SMTP_USER_NAME |
Authentication identity | Your provider’s SMTP username, often a complete email address or generated credential. |
DISCOURSE_SMTP_PASSWORD |
Authentication secret | The SMTP password, API-generated SMTP credential or provider-specific secret. |
DISCOURSE_SMTP_ENABLE_START_TLS |
Upgrade a plain connection to TLS using STARTTLS | true when the provider expects STARTTLS, as on the usual port 587 configuration. |
DISCOURSE_SMTP_FORCE_TLS |
Require an already encrypted TLS connection | Set true for a provider’s implicit-TLS submission service, commonly port 465. |
DISCOURSE_SMTP_DOMAIN |
Optional SMTP client/domain value | Only when the provider requires it; obtain the exact value from that provider. |
DISCOURSE_SMTP_OPENSSL_VERIFY_MODE |
Certificate verification behavior | Only when provider-specific guidance calls for it. Do not disable verification as a guess. |
Discourse’s defaults are an SMTP port of 25, plain authentication and STARTTLS enabled. Authentication is used when a username or password is present. Those defaults do not mean port 25 is the right submission port for your provider; many networks block outbound port 25 or providers reserve it for server-to-server traffic.
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Configure the variables in containers/app.yml
-
Open a root shell on the Discourse host and change to the installation directory:
cd /var/discourse -
Open the container configuration in your editor:
sudo editor containers/app.ymlUse the editor available on your system, such as
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Find the existing
env:block. Add or update the SMTP entries without creating a secondenv:key. A standard STARTTLS example is:env: DISCOURSE_SMTP_ADDRESS: smtp.example.com DISCOURSE_SMTP_PORT: 587 DISCOURSE_SMTP_USER_NAME: [email protected] DISCOURSE_SMTP_PASSWORD: "replace-with-secret" DISCOURSE_SMTP_ENABLE_START_TLS: true -
Replace every example value with the provider’s values. Quote a password when it contains YAML-significant characters, such as
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Save the file and check for common YAML mistakes: tabs instead of spaces, duplicate keys, a misplaced colon in an unquoted password and an
env:block nested under the wrong parent.
Do not add DISCOURSE_SMTP_FORCE_TLS: true to a normal STARTTLS setup unless your provider specifically requires both modes. For implicit TLS on port 465, use the provider’s documented combination, normally including DISCOURSE_SMTP_FORCE_TLS: true and the corresponding port.
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| Port | Transport behavior | Typical Discourse setting | When to use it |
|---|---|---|---|
| 25 | Traditional SMTP; may begin unencrypted and negotiate according to server policy | Discourse’s default port, with STARTTLS enabled by default | Only when your provider and network explicitly permit authenticated submission on 25. |
| 587 | Message-submission port that commonly starts plain and upgrades with STARTTLS | DISCOURSE_SMTP_PORT: 587 and DISCOURSE_SMTP_ENABLE_START_TLS: true |
The usual choice for authenticated application mail. |
| 465 | Implicit TLS: encryption begins as soon as the connection opens | Port 465 with DISCOURSE_SMTP_FORCE_TLS: true, following provider guidance |
When the provider documents implicit TLS rather than STARTTLS. |
| 2525 | Alternate submission port offered by some relays | Use the same TLS setting required by the provider, changing only the port | When port 587 or 25 is blocked or the provider recommends 2525. |
STARTTLS and implicit TLS are different protocols. Selecting port 465 while leaving a STARTTLS-only configuration, or selecting port 587 while forcing implicit TLS, commonly produces handshake errors or an immediate disconnect. Copy the provider’s current port-and-TLS pairing rather than trying combinations at random.
Load the new environment into the app container
Editing app.yml does not change the environment of an already running container. Recreate the app after saving the file:
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cd /var/discourse
./launcher destroy app
./launcher start appThe destroy command removes the app container, not the persistent Discourse data stored in the installation’s volumes. The start command creates a new container from the edited configuration. If your maintenance procedure calls for a rebuild, the documented alternative is:
cd /var/discourse
./launcher rebuild appA rebuild takes longer because the image and application layers may be rebuilt. Use the operation that fits your maintenance window and established backup process; in either case, ensure the newly created container receives the edited env values.
Set a sender address the relay will accept
SMTP authentication and sender authorization are separate checks. A relay can accept your username and password, then reject the message because the From address is not allowed. In Discourse, set the notification email address to a mailbox or verified sender permitted by the provider. If the service requires the sender to match the authenticated mailbox, use that exact mailbox address in the Discourse notification-email site setting.
Sender-domain verification may also require DNS records or a provider-side approval step. Complete those requirements before testing; changing only the SMTP password will not authorize an unverified domain.
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Send a controlled test
- After the container has been recreated or rebuilt, sign in as a Discourse administrator.
- Confirm the site’s notification email setting uses the permitted From address.
- Trigger one predictable message, such as a password-reset email for a test account or a notification from a private test topic.
- Check the recipient mailbox, including its spam or quarantine folder.
- Inspect Discourse’s email logs and the SMTP provider’s activity or rejection log. Record the exact timestamp, recipient, response code and error text before changing settings.
Test with a mailbox you control and one message at a time. A successful SMTP connection does not guarantee inbox placement: the relay may accept the message and later defer, bounce or filter it according to its own delivery and reputation policies.
Troubleshoot by symptom
The forum cannot connect to the SMTP host
- Verify that the hostname in
DISCOURSE_SMTP_ADDRESSresolves from the Discourse host. A hostname that resolves on your laptop may not resolve on the server because of different DNS settings. - Check outbound firewall rules and hosting-provider egress restrictions for the selected port.
- Confirm the port is the provider’s submission port, not an inbound service port intended for a mail server.
- Try the provider’s documented alternate submission port, such as 2525, when 25 or 587 is blocked. Change only one variable at a time and then recreate the container so the test uses the new value.
TLS handshake, certificate or protocol errors
- Match the encryption mode to the port: STARTTLS for a connection that upgrades, implicit TLS for a connection encrypted from byte one.
- Check the provider’s required TLS version and hostname. Certificate verification normally should remain enabled.
- Do not copy a provider-specific
DISCOURSE_SMTP_OPENSSL_VERIFY_MODEvalue from another service. Use it only when the provider’s current instructions explain why it is needed.
Authentication fails
- Re-enter the username exactly as issued; some services require the full email address while others issue a generated SMTP identity.
- Reset or regenerate the SMTP credential if the provider distinguishes it from the account’s web-login password.
- Check whether the account is allowed to submit through SMTP and whether the provider requires an additional domain value.
- After correcting a secret in
app.yml, recreate or rebuild the app; a running container keeps its old environment.
Authentication succeeds but the message is rejected
- Read the provider’s SMTP response for a sender-policy error.
- Set Discourse’s notification email to the authenticated mailbox or another sender explicitly verified by the provider.
- Verify the sending domain in the provider’s control panel and publish any required DNS records.
- Check recipient, content or rate-limit restrictions if the rejection is not sender-related.
Discourse reports success but no mail arrives
- Look at the provider’s delivery, defer and bounce records. “Accepted” means the relay took responsibility for processing, not that the recipient placed it in the inbox.
- Search spam, quarantine and server-side filtering at the recipient domain.
- Confirm the recipient address and inspect Discourse’s email log for a later bounce or retry.
- Send a second test to a different mailbox to distinguish a recipient-domain issue from a forum-wide configuration issue.
Why an existing SMTP service is usually preferable to running your own mail server
For a new community, Discourse support guidance generally favors an established SMTP service rather than operating a mail server yourself. A relay provides managed outbound connectivity, authentication, TLS support and delivery feedback. Running your own server adds DNS, reverse-DNS, IP reputation, abuse handling, queue management, bounce processing, certificate maintenance and monitoring responsibilities.
When evaluating services, compare the operational properties that affect a forum:
- Delivery reputation: whether the service separates customers appropriately and explains reputation controls.
- Authentication: password, generated SMTP credential, API-derived credential or other required identity.
- Ports and TLS: support for 587 with STARTTLS, 465 implicit TLS and any alternate port such as 2525.
- Sender verification: mailbox confirmation, domain verification and DNS requirements.
- Logs and bounce handling: searchable message events, deferrals, bounces and complaint signals.
- Regional availability: whether account creation, sending and data handling are available in your jurisdiction.
- Cost and limits: recurring fees, included volume, rate limits and overage rules as currently published by the provider.
One service discussed in Discourse configuration examples is Amazon SES, but the same evaluation criteria apply to any relay. Choose based on the provider’s current documentation and your forum’s sending volume, geography and compliance requirements rather than on a port number alone.
Security and maintenance practices
- Use a credential dedicated to the forum, with the least privilege your provider supports.
- Limit read access to
app.yml; it contains the SMTP secret alongside other deployment settings. - Keep a secure record of which sender domains and mailboxes are verified, so a staff change does not silently break notifications.
- Review delivery and bounce logs after changing domains, credentials or providers.
- Rotate the SMTP credential through a planned maintenance window: update
app.yml, recreate or rebuild the app, send a controlled test and then revoke the old credential. - Document the selected port and TLS mode. Future administrators should not infer that port 465 is interchangeable with STARTTLS on 587.
Configuration checklist
DISCOURSE_SMTP_ADDRESScontains the provider’s exact hostname.DISCOURSE_SMTP_PORTmatches the provider’s submission service.- STARTTLS or implicit TLS is configured for that port, not the other mode.
- Username and password are current, correctly quoted and stored only in the protected configuration.
- Any required domain or provider-specific OpenSSL setting comes from current provider instructions.
- The notification From address is an authorized mailbox or verified sender.
- The app container was recreated or rebuilt after the file changed.
- A test message was checked in Discourse logs, the provider log and the recipient mailbox.
The Bottom Line
For most self-hosted Discourse forums, place the provider’s SMTP hostname, credentials, port 587 and STARTTLS setting under env in /var/discourse/containers/app.yml. Recreate or rebuild the app container, use an authorized notification address and verify the result in both Discourse and the relay’s logs. Port 465 is an implicit-TLS alternative, while 2525 can help when a provider offers it or another submission port is blocked.
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