A send-only iRedMail server is not a separate iRedMail product mode. It is an iRedMail installation configured so local applications or approved client networks can submit messages, while Postfix forwards every message to an authenticated upstream smarthost and the server does not accept public inbound delivery. The reliable way to build it is to configure Postfix’s relayhost and SASL maps, preserve any existing iRedMail database lookups, restrict relay access with both Postfix and iRedAPD, and then verify delivery and rejection behavior from controlled tests.
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What “send-only iRedMail” means
iRedMail supplies a complete mail-server stack, but there is no documented switch called “send-only.” The behavior comes from Postfix configuration and network policy. In this design, iRedMail remains useful for local mail submission, account management, logging, and administration, but outbound messages are handed to another SMTP service instead of being delivered directly to recipient domains.
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The practical flow is:
- An application or approved client submits a message to the Ubuntu host.
- Postfix accepts it only from a permitted local address, LAN, or authenticated path.
- Postfix selects the configured upstream relay, commonly on submission port
587. - Postfix authenticates with credentials stored in a SASL password map.
- The upstream service performs final delivery.
“Send-only” therefore describes an operating policy, not an iRedMail edition. Whether this arrangement is preferable to a standalone Postfix relay depends on whether you need iRedMail’s surrounding administration and mail-stack features; no universal winner is established.
Before you change the server
Gather the values the relay requires
- An Ubuntu server with iRedMail already installed, or a planned iRedMail installation that you will configure after the installer completes.
- The upstream SMTP hostname, port, username, and password supplied by your relay service. The examples use port
587. - The sender address or domains that the upstream service permits. Some relays require a separate credential for each sender or domain.
- The IP ranges that are allowed to submit mail to this server. Keep this list as small as possible.
Do not assume that a setting shown in an older guide exists with the same default in your release. Record the installed iRedMail and Postfix versions, then verify the active values with Postfix itself before editing files.
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Take a configuration snapshot
Run postconf -n and save its output before making changes. This gives you a restorable record of non-default Postfix settings and helps reveal whether iRedMail already uses a MySQL or MariaDB-backed lookup for sender-dependent relaying. Also run:
postconf sender_dependent_relayhost_mapsIf this command returns an existing value, treat it as part of the iRedMail design. Replacing it with a new hash map without understanding the current lookup can silently break sender routing.
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One upstream relay for all senders
A global relayhost is the simplest arrangement. Every message accepted by this Postfix instance is sent to the same upstream host, and one SASL map supplies the credentials. This is appropriate when one account or relay endpoint handles all outbound mail.
Different relays by sender or domain
Postfix can select an upstream relay according to the envelope sender. Enable smtp_sender_dependent_authentication and use sender_dependent_relayhost_maps. The map can associate a sender or domain with a relay endpoint, while the SASL password map holds the corresponding credentials. A default relayhost can remain as a fallback when no sender-specific entry matches.
| Requirement | Configuration approach | Important caution |
|---|---|---|
| All outbound mail uses one account | Global relayhost plus one SASL password map |
Every accepted sender must be authorized by that upstream account. |
| Senders or domains use different accounts | sender_dependent_relayhost_maps and sender-dependent SASL authentication |
Confirm the existing iRedMail lookup before replacing or extending it. |
| Only local software submits mail | Loopback-only listening and a tightly scoped mynetworks |
Remote clients will not be able to submit unless you deliberately expose an appropriate authenticated interface. |
| Applications on a private LAN submit mail | Listen on the required LAN address and allow only that network | The same network must also be recognized by iRedAPD. |
Configure a global authenticated relay
1. Set the relay host
Edit the active Postfix main.cf and set the upstream endpoint in brackets. Brackets prevent Postfix from performing an MX lookup for the relay hostname:
relayhost = [smtp.example.com]:587Replace the example hostname with the relay service’s real submission hostname. Keep the brackets and port syntax exactly as required by Postfix. The examples use port 587; use another port only when your relay provider documents it.
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Create the password-file referenced by Postfix. Its key must match the relay endpoint, including the bracketed host and port, and its value contains the upstream username and password:
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[smtp.example.com]:587 relay-user:relay-passwordUse the actual credentials issued for your relay account. Keep this file readable only by the account that needs it, and do not place credentials in application source code or publish the file in a backup repository.
3. Hash the map and enable authentication
Build the database map with postmap, then reference it from main.cf:
postmap /etc/postfix/sasl_passwd
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymousThe filename may differ if your installation uses another path; the critical requirement is that the path in smtp_sasl_password_maps matches the file you hashed. After changing Postfix settings, reload the service using the method appropriate to your Ubuntu installation, then inspect the active configuration with postconf -n.
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4. Consider transport security as a provider requirement
The relay examples establish authentication and use submission port 587, but they do not establish one universal TLS setting for every provider or Postfix release. Check the upstream service’s current requirements and your installed Postfix documentation before enabling or enforcing a specific TLS policy. A successful TCP connection alone does not prove that the relay accepted authenticated mail.
Configure sender-dependent relaying
Understand iRedMail’s existing map first
iRedMail deployments commonly keep sender-dependent relay information in MySQL or MariaDB. One path is:
proxy:mysql:/etc/postfix/mysql/sender_dependent_relayhost_maps.cfThat path is an example of the lookup form, not a guarantee for every release. Always inspect the live value with:
postconf sender_dependent_relayhost_mapsIf a database lookup is already configured, add or change the corresponding records through the mechanism your installation uses instead of overwriting the parameter with a hash file. An iRedAdmin per-domain relay control applies to received mail in the iRedMail forum explanation; it should not be assumed to configure outbound sender routing.
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Use a hash map when your installation has no existing database map
For a standalone hash-map arrangement, enable sender-dependent authentication and point Postfix at a relayhost map:
smtp_sender_dependent_authentication = yes
sender_dependent_relayhost_maps = hash:/etc/postfix/relayhost_mapsA domain-to-relay example maps a domain to an upstream relay on port 587:
example.org [smtp.example.com]:587Hash the file after editing:
postmap /etc/postfix/relayhost_mapsPopulate the SASL password map with credentials that correspond to the sender or relay key expected by your Postfix lookup. The Postfix SOHO guidance describes searching the SASL map by sender before destination and using the default relayhost as a fallback. Test each sender that matters; a syntactically valid map can still select the wrong credentials if its key does not match the envelope sender.
Restrict who can submit and relay
Control Postfix’s trusted networks
Postfix uses mynetworks to define networks trusted to relay. Set it to loopback only when applications run on the same server, or to the smallest private LAN range that genuinely needs access. Do not add a broad public range merely to make a test client work.
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Keep iRedAPD’s network list in sync
The iRedMail forum guidance states that the permitted network must also be added to iRedAPD’s MYNETWORKS setting in /opt/iredapd/settings.py. Verify both lists on the actual server. If Postfix trusts a network but iRedAPD does not, local submissions can fail; if the lists are broader than intended, you may grant relay access to clients that should be blocked.
Limit listening interfaces
Use Postfix’s inet_interfaces to bind only where clients exist. A loopback-only configuration is suitable when all senders are local applications. If approved clients are on a private LAN, bind the required LAN address as well, while keeping the network allow-list narrow. Binding publicly reachable interfaces without an equally strict access policy exposes the SMTP service to probing and abuse.
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Make the server outbound-only in practice
Outbound-only behavior is the combination of relay routing, client restrictions, and network exposure. Do not describe the host as send-only merely because relayhost is set: Postfix could still accept unauthorized submissions or listen on an unintended interface.
- Ensure every accepted message has a path to the configured upstream relay.
- Do not authorize arbitrary external networks in
mynetworks. - Use loopback-only or LAN-only
inet_interfaceswhen public SMTP reception is unnecessary. - Review any firewall or cloud security-group rule that exposes SMTP service ports, and remove access that the design does not require.
- Do not assume an iRedAdmin received-mail relay setting changes outbound behavior; sender-dependent Postfix settings control outbound selection.
No universal DNS recipe for an outbound-only host is established, so DNS changes should follow your domain and upstream relay provider’s current instructions rather than an assumed template.
Apply changes without losing iRedMail integration
Inspect, edit, then re-check
- Capture
postconf -nand the currentsender_dependent_relayhost_mapsvalue. - Edit only the relay, SASL, sender-dependent, network, and interface settings required for your chosen design.
- Hash every changed
hash:map withpostmap. - Reload Postfix using your system’s service-management command.
- Run
postconf -nagain and confirm that the intended values are active. - Check that an existing
proxy:mysql:sender map was not accidentally replaced.
Keep a rollback path
Retain the original configuration snapshot and the pre-change copies of any map files. If Postfix refuses to reload, restore the last known-good values before troubleshooting the relay credentials. A failed reload is safer than continuing with a partially edited configuration.
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Test a permitted local sender
Send one message from a local application or an address in an explicitly allowed network. Use a recipient mailbox you control. Verify that the message leaves through the upstream relay rather than being delivered directly by examining the Postfix mail log and the relay’s delivery record, if it provides one.
Test every sender-dependent route
If you configured per-domain or per-sender maps, send a separate test for each mapping. Confirm that the selected endpoint and credential match the envelope sender. Also test a sender with no specific entry to verify that the documented default relayhost fallback behaves as intended.
Test an unauthorized external host
From a host outside the permitted networks, attempt an SMTP relay operation without authorization. The server should reject the attempt. This test is essential: successful delivery from the approved client proves only that your credentials work, not that the server is closed to abuse.
Read logs by symptom
- Authentication failure: check the SASL map key, username, password, and whether the hashed database is newer than the source file.
- Wrong relay selected: inspect sender-dependent map precedence and confirm that an existing MySQL lookup was not overwritten.
- Local client rejected: compare its address with
mynetworks, iRedAPD’sMYNETWORKS, and the address bound byinet_interfaces. - External host relays successfully: stop testing, remove the excessive network allowance, and repeat the rejection test before sending production mail.
Common configuration mistakes
Treating iRedAdmin’s received-mail relay setting as outbound routing
The iRedMail forum explanation distinguishes relay settings for received mail from sender-dependent outbound routing. Use Postfix’s relayhost or sender-dependent maps for outgoing messages.
Replacing a database map with a hash map
On an iRedMail server that already uses a proxy:mysql: map, assigning sender_dependent_relayhost_maps to a new hash file can disconnect the administrative data from Postfix. Inspect first, then modify the existing data path or choose a design that preserves it.
Hashing the wrong file
Postfix reads the generated map named by the hash: parameter. Running postmap against a differently named file, or editing the source without regenerating its database, leaves Postfix using stale credentials or routes.
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Allowing a whole internet-facing range
mynetworks is an access-control decision, not a connectivity workaround. Add only the loopback address or private client ranges that require submission, and mirror the intended list in iRedAPD.
Assuming port 587 explains every failure
The examples use port 587, but a provider may require different authentication or transport policies. Confirm the endpoint and security requirements supplied for your account instead of changing ports randomly.
When a standalone Postfix relay may be simpler
A send-only workload can be implemented with iRedMail plus outbound-only settings or with a smaller standalone Postfix relay. iRedMail may be justified when you need its integrated account, policy, and administration components. A standalone relay can reduce the number of moving parts when no mailbox hosting or iRedMail administration is needed. No measured performance, security, or maintenance comparison is provided, so choose based on the features you actually require and the operational expertise available to maintain them.
Operational checklist
- Upstream hostname, port, account, and permitted senders are documented.
relayhostuses bracketed host syntax and the intended port.- SASL authentication is enabled, the password map is referenced, and the map has been regenerated with
postmap. postconf sender_dependent_relayhost_mapshas been checked before any sender-routing edit.- Per-sender authentication is enabled only when sender-specific routing is required.
mynetworks, iRedAPD’sMYNETWORKS, andinet_interfacesagree with the actual clients.- A permitted test message succeeds through the upstream relay.
- An unauthorized external relay attempt is rejected.
- Logs show the expected upstream handoff and no repeated authentication or routing errors.
What success looks like
After the configuration is complete, local or explicitly permitted clients can submit mail, Postfix authenticates to the selected upstream relay, and messages are handed off there. The Ubuntu host is not an open relay, does not expose unnecessary listening interfaces, and retains any iRedMail database-backed sender routing that was already in use. Re-run the verification commands after upgrades, because exact defaults and integration details can vary between iRedMail and Postfix releases.
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Does iRedMail have a built-in “send-only” mode?
No separate mode is documented. Send-only behavior is created by configuring Postfix to relay outbound mail through an authenticated smarthost and restricting who can submit or connect.
Can I use iRedAdmin to choose the outbound relay for a domain?
Do not assume so. The cited iRedMail forum guidance says the per-domain relay setting applies to received mail; outbound selection uses Postfix relayhost or sender-dependent relayhost maps.
Why must I check sender_dependent_relayhost_maps before editing?
iRedMail may already use a MySQL or MariaDB lookup, such as a proxy:mysql map. Overwriting that parameter with a hash file can disconnect Postfix from the existing administrative data.
How do I prove the server is not an open relay?
Send a test from an allowed client, then attempt an unauthenticated relay from a host outside the permitted networks. The external attempt must be rejected, and the logs should show no unauthorized upstream handoff.
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