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A “send-only” iRedMail server is not a separate iRedMail installation mode. It is an iRedMail/Postfix configuration that accepts messages from approved local applications or networks, sends every outbound message to an authenticated smart host, and does not accept Internet mail for local delivery. The essential settings are a Postfix relayhost (or sender-dependent relay map), a SASL password map, and tight limits on which clients may submit mail.
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This guide shows how to apply that pattern on an existing Ubuntu iRedMail server, how to choose between one global relay and per-sender routing, how to preserve iRedMail’s existing database-backed maps, and how to prove that outbound delivery works without turning the host into an open relay.
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What “send-only” means in iRedMail
iRedMail normally bundles Postfix with mailbox, filtering, authentication and administration components. It does not provide a documented switch named “send-only.” Instead, you configure the Postfix transport so the server behaves as a submission relay:
- Local applications, loopback clients or explicitly approved LAN addresses can submit messages.
- Postfix forwards those messages to an upstream authenticated relay, also called a smarthost.
- The Ubuntu host is not an Internet destination for the domains it sends from.
- Unauthorised outside hosts cannot use it to relay.
“No inbound delivery” has two parts. Network access to SMTP must be restricted, and your public DNS and mail architecture must ensure that incoming mail for the domain is handled elsewhere. Merely setting a relay host does not, by itself, remove every inbound or local-delivery path.
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Plan the topology before changing Postfix
Global outbound relay
Use one relay for all senders when every application and domain can use the same upstream account and endpoint. This is the simplest and least error-prone arrangement. Postfix reads one relayhost value and one SASL credential map.
Sender-dependent relays
Use sender-dependent routing when different envelope senders or domains must use different upstream accounts or endpoints. Postfix can enable smtp_sender_dependent_authentication and consult sender_dependent_relayhost_maps. A sender map can select a relay for a domain or address, while the SASL password map supplies the matching credentials.
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|---|---|---|
| Configuration | relayhost = [smtp.example.com]:587 plus one credential map |
sender_dependent_relayhost_maps, sender authentication and matching credentials |
| Best fit | One provider account for all applications | Several domains, accounts or relay endpoints |
| Operational risk | Fewer moving parts | More lookup and database entries to audit |
| iRedMail caution | Usually independent of iRedMail’s per-domain database maps | Check the active map first; replacing an existing MySQL lookup can break current routing |
Port 587 appears in common Postfix relay examples and is the normal submission endpoint for many providers. Use the port and encryption requirements specified by your relay service; do not assume that every provider uses the same port.
Prerequisites and a safe change window
- An Ubuntu server with iRedMail already installed and working for local submission.
- Administrative access through
sudoor a root shell. - An account with your upstream relay, its SMTP hostname, port, username and password or token.
- A list of the applications or network ranges that are allowed to submit mail.
- A maintenance window long enough to reload Postfix and send several test messages.
Do not begin by deleting iRedMail configuration files. First capture the active configuration and make a copy of every file you will edit:
sudo postconf -n > /root/postconf-before-send-only.txt
sudo postconf sender_dependent_relayhost_maps
sudo cp -a /etc/postfix/main.cf /etc/postfix/main.cf.before-send-only
sudo cp -a /etc/postfix/master.cf /etc/postfix/master.cf.before-send-onlyThe command output is the authority for your installed Postfix build. Paths and lookup methods can differ between iRedMail releases, so use the values shown on this server rather than copying a path from another installation.
Configure a single authenticated relay
1. Set the relay host in main.cf
Open the active Postfix configuration:
sudo editor /etc/postfix/main.cfAdd or change the following settings, replacing the example host and port with the values supplied by your relay service:
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relayhost = [smtp.example.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymousThe square brackets around the host are significant: they tell Postfix to connect to that exact host instead of performing an MX lookup for it. Keep only one active relayhost line. If an existing iRedMail setting already provides one, edit that line rather than creating a conflicting duplicate.
The smtp_sasl_security_options = noanonymous setting prevents anonymous SASL mechanisms. Your provider may require additional TLS or certificate settings; retain iRedMail’s existing secure transport configuration and follow that provider’s current Postfix instructions for any extra requirements.
2. Create and protect the SASL password map
Create the source password file with the relay endpoint as the key. The key must match the bracketed host and port used in relayhost:
sudo editor /etc/postfix/sasl_passwd[smtp.example.com]:587 relay-user:strong-passwordUse a generated password or provider token rather than a mailbox password that is reused elsewhere. Restrict the clear-text file to root and the Postfix processes that need it, then build the hashed map:
sudo chown root:root /etc/postfix/sasl_passwd
sudo chmod 600 /etc/postfix/sasl_passwd
sudo postmap /etc/postfix/sasl_passwd
sudo chmod 600 /etc/postfix/sasl_passwd.dbAfter postmap, both the source file and its .db file should be treated as credentials. Back them up only in an encrypted location. If you change the password later, edit the source file and run postmap again; Postfix reads the database, not the un-hashed text, during delivery.
3. Validate and reload
Check the syntax before asking Postfix to use the new map:
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sudo postfix check
sudo systemctl reload postfix
sudo postconf relayhost smtp_sasl_auth_enable smtp_sasl_password_maps smtp_sasl_security_optionsA successful reload does not prove authentication or delivery. It only proves that Postfix accepted the configuration. Perform the message test described later and inspect the mail log for the upstream server’s response.
Configure different relays for different senders
Inspect iRedMail’s current map before editing
iRedMail installations commonly use a database-backed sender-dependent map. Check the active value:
sudo postconf sender_dependent_relayhost_mapsAn output such as proxy:mysql:/etc/postfix/mysql/sender_dependent_relayhost_maps.cf means Postfix is already querying MySQL or MariaDB. Do not replace that line with a hash file merely because a tutorial uses one; doing so can silently remove existing sender routing.
Some installations expose the database-backed map through iRedMail’s administration data, while others have local customisations. Use the schema and map query already configured on this server, and take a database backup before changing rows. A per-domain relay setting in the iRedMail administration interface concerns received mail in some iRedMail versions; it should not be assumed to select the relay used for outbound messages.
Use a hash map only when no managed map is active
If postconf shows no existing sender-dependent map and you deliberately want a simple file-based setup, create a map such as:
example.org [smtp.example.com]:587
alerts.example.net [smtp.other.example]:587Build it and point Postfix at it:
sudo postmap /etc/postfix/relayhost_maps
sudo postconf -e 'sender_dependent_relayhost_maps = hash:/etc/postfix/relayhost_maps'
sudo postconf -e 'smtp_sender_dependent_authentication = yes'
sudo systemctl reload postfixThe lookup keys and fallback order matter. Postfix’s sender-dependent authentication documentation describes checking sender-specific credentials before destination-based credentials and using a default relay when no sender match exists. Keep a default relayhost if unmatched senders must still have a defined route, and test both a matching and a non-matching sender.
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Keep credentials aligned with sender routes
The password map must contain entries that match the lookup order used by your Postfix version and configuration. Depending on the active settings, Postfix can look up a sender address, a sender domain or the relay endpoint. Inspect the installed Postfix documentation and existing smtp_sasl_password_maps value rather than guessing the key. After every change, run postmap on the password file and send a message from each configured sender.
Restrict who is allowed to submit
Set Postfix mynetworks
mynetworks defines networks that Postfix treats as trusted clients. For a host used only by local applications, keep the trust boundary on loopback. For a separate application server on a private LAN, add only that server’s fixed address or the smallest required subnet. Review the active value with:
sudo postconf mynetworksEdit /etc/postfix/main.cf only after documenting the current list. Never add an entire public address range or an interface that is reachable from the Internet simply to make a test pass.
Keep iRedAPD’s network list in sync
iRedMail’s policy daemon also has a network allow-list. The iRedAPD setting is in /opt/iredapd/settings.py and is commonly named MYNETWORKS. If a legitimate client is in Postfix’s mynetworks but not in iRedAPD’s list, policy checks can still reject it. If it is in iRedAPD’s list but Postfix does not trust it, Postfix can reject it first. Verify both lists and restart or reload the relevant service using the procedure documented for your installed iRedAPD release.
Choose the listening interfaces
When every sender runs on the same machine, set Postfix to listen on loopback only, using the syntax supported by your installed configuration:
inet_interfaces = loopback-onlyFor a private application network, listen on loopback and the specific private interface instead of every address. Confirm the result with postconf inet_interfaces and an operating-system socket check. An Internet-facing listener is not necessary for a local-only relay and increases the chance of accidental abuse.
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Prevent inbound mail from reaching the send-only host
Outbound relaying and inbound delivery are separate Postfix decisions. To make the role genuinely send-only:
- Restrict SMTP listening with
inet_interfacesand your firewall so untrusted Internet hosts cannot connect. - Allow only the loopback or private client addresses that actually submit mail.
- Ensure public DNS does not direct inbound mail for your production domain to this host when another system is responsible for receiving it.
- Review iRedMail’s local domains and aliases so you know which addresses Postfix still considers local. Do not remove local-domain settings blindly; applications may depend on them for submission.
- Test from an external machine that an unauthenticated connection cannot relay and that the host is not advertised as the destination for incoming mail.
If the same domain must both send through this Ubuntu host and receive elsewhere, treat DNS, mailbox hosting and outbound envelope routing as separate designs. A relayhost setting cannot move incoming mail to another service.
Send a controlled test message
Test locally first
Submit a message from the same host using the local sendmail-compatible interface or the application that will use the relay. Use a recipient mailbox you control and a distinctive subject. Immediately watch the mail log:
sudo tail -f /var/log/mail.logOn systems that route service logs through journald, use:
sudo journalctl -u postfix -fA successful transaction normally shows Postfix connecting to the configured upstream host and receiving a successful SMTP response. The exact log wording varies by Postfix version and provider, so evaluate the delivery status and queue ID rather than looking for one fixed line.
Test every sender-dependent path
For a sender map, repeat the test with one sender that has a specific route and another that should use the default. Confirm in the log that each message connects to the intended relay and authenticates with the intended account. A message accepted locally but left in the queue is not a successful send-only test.
Test rejection from an untrusted host
From a machine outside the approved networks, attempt an SMTP submission without authentication. The server should refuse relay or refuse the connection according to your access policy. Do not use a real recipient for this test. If the host accepts and queues mail from an untrusted source, stop the service’s external exposure, correct mynetworks, iRedAPD’s MYNETWORKS, listener interfaces and firewall rules, then repeat the test.
Troubleshoot the common failure modes
| Symptom | Likely cause | What to check |
|---|---|---|
| “SASL authentication failed” | Wrong credential, wrong map key or stale database file | Confirm the exact bracketed host and port in sasl_passwd, run postmap again, check file permissions and inspect the active smtp_sasl_password_maps value. |
| Upstream says relay access denied | The provider does not permit the account, sender or destination | Verify the relay account, allowed sender identities and provider port. This is an upstream policy response, not proof that local Postfix is broken. |
| Message remains queued | Network, DNS, TLS or upstream availability problem | Use the queue ID in the mail log, confirm that the configured relay host resolves and is reachable, and read the complete deferred-delivery reason. |
| Per-domain route is ignored | An existing MySQL map was overwritten, or the lookup key does not match | Run postconf sender_dependent_relayhost_maps, restore the original managed map if necessary, and verify whether the map expects an address or domain key. |
| Local client is rejected | Its address is absent from one of the allow-lists | Compare mynetworks with iRedAPD’s MYNETWORKS, check the client’s actual source address and reload the service after editing. |
| External host can relay | Overly broad trusted networks or an exposed listener | Remove public ranges from mynetworks, narrow inet_interfaces, block unwanted SMTP access at the firewall and retest from outside. |
| Postfix will not reload | Syntax error or invalid map reference | Run postfix check, inspect the service journal, and compare the changed lines with the backup made before the change. |
Operate the relay securely
- Keep
/etc/postfix/sasl_passwdand its database readable only by the required account or root. - Use a relay-specific credential that can be revoked independently of user mailboxes.
- Rotate the credential at the upstream provider, update the map, run
postmap, reload Postfix and send a test. - Monitor the mail log and queue for unexpected volume, repeated authentication failures or deferred messages.
- Record the approved client addresses and review them whenever an application is retired or moved.
- Preserve a known-good copy of
main.cfand the active map definitions so a failed change can be reversed quickly.
To inspect queued mail without altering it, use:
sudo postqueue -pDo not purge the queue as a first response. Capture queue IDs and log messages first; deleting queued mail can destroy evidence and legitimate messages.
Global relay or standalone Postfix?
For a server that already runs iRedMail and needs local applications, keeping iRedMail and configuring Postfix as an outbound relay avoids introducing a second mail stack. A standalone Postfix relay can have a smaller footprint, but deciding between the two depends on the services you already use, your update process and whether iRedMail’s filtering and administration components are needed. The available configuration evidence does not establish a universal winner. Treat it as an architectural trade-off, not as a feature guarantee of a particular iRedMail release.
Quick Recap
Final verification checklist
postconf -nshows the intendedrelayhostor sender-dependent map.- The relay endpoint in the password file exactly matches the configured host and port, including brackets where required.
postmaphas been run after the latest credential change and both map files have restrictive permissions.- Postfix’s
mynetworksand iRedAPD’sMYNETWORKScontain every approved client and no public range. inet_interfacesand the firewall expose SMTP only where clients require it.- A local test reaches the upstream relay and arrives at the recipient.
- Each sender-dependent route has been tested, including the fallback route.
- An external unauthenticated host cannot relay through the server.
- Public DNS and the receiving-mail design do not send inbound domain mail to this send-only host.
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