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What the three-layer design protects
Outgoing SMTP abuse usually occurs in one of two ways. A compromised process opens a direct connection to a remote mail server on TCP port 25, bypassing your MTA entirely, or an application submits locally at a rate that overwhelms your queue and damages the server’s reputation. A single Exim setting cannot reliably stop both paths.
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Use these layers together:
- Network egress control: deny direct outbound TCP/25 except through an explicitly approved relay path. This is the boundary that still works if an attacker never contacts Exim.
- Local submission control: permit Exim, Mailman and root (or specifically approved service accounts) to connect to remote SMTP servers. On cPanel, SMTP Restrictions is designed for this purpose.
- Exim policy: use ACLs to reject suspicious SMTP commands and the
ratelimitcondition to throttle abusive activity across connections and over time.
The result is not simply “port 25 is closed.” Applications continue to deliver mail by authenticating to the local Exim service, while Exim alone sends onward through the approved smarthost.
Decide what traffic is allowed before changing Exim
Separate submission from server-to-server delivery
Port 25 is traditionally used for SMTP server-to-server delivery. Submission clients should instead use authenticated STARTTLS on port 587 or SMTPS on port 465. The Debian Project recommends those submission ports instead of plain SMTP on port 25 for practical deployments. Your policy should therefore distinguish:
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| Connection | Purpose | Recommended policy |
|---|---|---|
| Application to local Exim | Message submission from a website, game panel, cron job or service | Allow only the local listener and approved credentials; require the application to use Exim rather than opening its own remote socket. |
| Exim to smarthost | Outbound handoff by the MTA | Allow the smarthost’s hostname or IP and its submission port, normally 587 or 465, with authentication configured in Exim. |
| Unapproved process to the Internet on TCP/25 | Direct delivery or abuse bypassing Exim | Deny at the VPS or provider egress boundary. |
| Exim to arbitrary Internet hosts on TCP/25 | Direct delivery without a relay | Deny unless your operational policy explicitly requires direct delivery and you can restrict it to the MTA. |
Record the approved relay hostname, destination port, authentication method, sender domains and expected message volume. Those decisions become the firewall exception, Exim route and ACL keys. Do not begin with a rate number copied from another server; Exim’s specification does not define one universal safe threshold.
Block direct outbound port 25 at the VPS boundary
Use the provider control when available
Many VPS providers block outbound TCP/25 by default. If yours does, leave that block in place and configure Exim to use a smarthost. If it does not, create an egress rule that rejects TCP/25 for the server, while permitting only the approved relay path. A provider-level rule is preferable because a local process cannot remove it after a compromise.
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Keep the exception narrow
- Permit only the smarthost address or addresses you actually use.
- Permit only the smarthost’s configured submission port, such as 587 or 465.
- Do not create a broad “allow all SMTP” exception for the entire VPS.
- If the relay publishes multiple addresses, include the documented set and monitor for changes.
A port-25 egress block can break old applications that contain a remote SMTP server and port 25 in their settings. Plan to change those applications to the local Exim host or to authenticated submission before enforcing the rule.
Enable local submission restrictions in cPanel/WHM
Turn on SMTP Restrictions
- Sign in to WHM with an administrator account.
- Open Security Center → SMTP Restrictions.
- Enable the restriction and save the change.
- Re-test any service that sends mail, including websites, scheduled jobs and mailing lists.
cPanel documents that this setting allows only the mail transport agent, Mailman and the root user to connect to remote SMTP servers. Ordinary users and services therefore cannot bypass Exim by opening their own outbound SMTP connection. Applications should submit to the cPanel server as their SMTP host and supply a valid address and password.
Add explicit IP policies when necessary
For additional control, cPanel provides IP deny lists at Service Configuration → Exim Configuration Manager → Access Lists. Use these lists for known abusive or untrusted sources, not as a substitute for the outbound egress block. An IP list affects Exim’s behavior; it does not stop a process that bypasses Exim and connects directly to the Internet.
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Account for service identities
Inventory every legitimate sender before enabling the restriction. A web application may run as a per-account Unix user, while a monitoring daemon may run under its own service identity. Give each sender one of two supported paths: submit through Exim using authenticated SMTP, or be explicitly approved as a local service that Exim can identify. Avoid granting broad root privileges merely to preserve an old mail script.
Configure Exim to relay through a smarthost
Use an alternate submission port
In a Debian Exim smarthost configuration, an alternate port is written after a double colon. A relay on port 587 is represented conceptually as:
smarthost.example::587The hostname and port select the route; authentication is configured separately in Exim’s credential section. Store relay credentials with permissions that prevent application users from reading them, and use the TLS mode required by the relay. Port 587 normally means authenticated SMTP with STARTTLS; port 465 means SMTPS with TLS established when the connection opens.
Align the route with the firewall
Do not configure Exim for port 587 while the egress policy permits only port 465, or vice versa. The route, credential stanza and provider rule must describe the same destination. If the relay has both names and addresses, verify that DNS resolution and the firewall exception cover the values Exim will actually use.
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Make applications use Exim
Change each application to submit to the local Exim listener, using the account’s SMTP credentials and the server name appropriate for that installation. Do not leave a hard-coded remote host on port 25 in a plugin, environment variable or scheduled script. A local submission test should produce an Exim log entry followed by a smarthost handoff; it should not show the application establishing an external TCP/25 session.
Use Exim ACLs to reject and throttle abuse
Where ACLs apply
Exim ACLs are declared under begin acl. They can control SMTP commands from network clients and local SMTP submissions made through Exim’s -bs interface. Place a check at the stage that matches the policy you want to enforce:
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- MAIL: reject or delay a sender before recipients are accepted.
- RCPT: apply recipient, domain or per-identity policy.
- DATA: inspect the message transaction after recipients have been accepted.
- Non-SMTP submission: protect locally generated messages that enter through a non-network path, where appropriate for your configuration.
Log each decision. A clear policy message and a corresponding log record let you distinguish an intentional abuse rejection from a DNS, TLS or relay-delivery failure.
Understand the newer ratelimit condition
The Exim ratelimit ACL condition measures activity across concurrent and sequential connections. That makes it suitable for detecting an account or service that opens many short sessions, a pattern that older per-session controls do not capture. Older smtp_ratelimit_* settings are session-oriented and should not be treated as an equivalent global quota.
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A conceptual ACL pattern is:
begin acl
acl_check_mail:
deny ratelimit = 100 / 1h / strict / $authenticated_id
message = submission rate exceeded
acceptThis means “deny when the authenticated identity reaches the illustrated rate,” but it is not a universal copy-and-paste configuration. The appropriate key, ACL hook, authentication variable and generated macros differ between standalone Exim and panel-managed builds. If unauthenticated local submissions are possible, $authenticated_id may be empty or unsuitable; choose a key that reliably separates the service or account you intend to limit.
The value of 100 messages per hour above is only an example threshold. Derive a production limit from normal application volume, alerting requirements, queue behavior and the relay’s own policy. Start in a logging or deliberately low-impact test mode where your build supports it, then tune the limit after observing legitimate bursts such as password resets or game-server notifications.
Choose the rejection point carefully
Rejecting at MAIL stops the transaction early and avoids accepting recipients that cannot be delivered. A DATA-stage check can incorporate message-level information but may consume more resources before rejection. Whichever stage you choose, make the policy message clear enough for the submitting application and log the identity, decision and counter key without exposing credentials.
Validate generated configuration before reload
- Render the effective configuration. Panel-managed Exim installations generate files and macros; inspect the rendered result rather than assuming that an edited template is active.
- Run Exim’s syntax check. Do this before reloading. A malformed ACL or router can stop Exim from accepting mail.
- Review the route. Confirm the smarthost name, alternate port, TLS requirement and authentication lookup are all present in the effective configuration.
- Reload only after validation. Keep an administrative session available so you can restore the previous configuration if the reload fails.
- Watch the log during a controlled submission. Verify that Exim accepts the message and records a handoff attempt to the intended relay.
Do not assume that a standalone Debian recipe applies unchanged to cPanel. Panel updates can regenerate Exim files, and the supported customization points and macro names may differ.
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Verification and failure testing
Test the blocked path
From an unapproved service account, attempt a controlled outbound TCP/25 connection to a test destination that you administer or are authorized to use. The attempt should be rejected by the egress policy. Repeat after a firewall or provider change; an old connection can make a newly blocked rule appear ineffective.
Test the approved path
Submit a message through the local Exim listener with valid application credentials. Confirm all of the following:
- The application connects to the local submission endpoint, not a remote host on port 25.
- Exim records an accepted submission with the expected authenticated identity.
- Exim opens the smarthost connection on the configured submission port.
- The relay accepts the handoff and the message leaves the queue.
Exercise the rate policy
In a test window, send enough controlled submissions to cross the configured threshold. Verify that Exim returns the policy message, records the rate-limit decision and leaves unrelated identities unaffected. Then test queue recovery and legitimate alerts after the limit is restored.
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Troubleshoot common breakages
Applications report connection refused or timeout
First determine whether the application is still targeting a remote port 25. If so, change it to the local Exim host or to the approved authenticated submission endpoint. If it already uses Exim, inspect the local listener, credentials and SMTP authentication policy before changing the firewall.
Exim accepts mail but cannot deliver it
Check the effective smarthost route, alternate port, DNS result, TLS negotiation and credential lookup. A successful local submission proves only that Exim accepted the message; it does not prove that the relay connection or authentication succeeded. The Exim log should identify whether the failure occurred during connection, TLS, authentication or remote rejection.
Legitimate bursts are rejected
Review the rate key and window. A key that groups several applications together can exhaust the quota unexpectedly, while a key that is too granular may fail to contain one compromised account. Increase or separate limits based on measured legitimate volume rather than deleting the ACL. Keep alerting in place so a higher limit does not hide abuse.
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SMTP Restrictions breaks a service
Identify the Unix user or daemon that opened the remote connection. Reconfigure it to authenticate to Exim, then retest. Avoid bypassing the restriction globally; its purpose is to make Exim the single controlled egress point.
The firewall appears to do nothing
Check the provider-level rule, local firewall order, IPv4 versus IPv6 behavior and existing long-lived connections. Test both address families if the VPS has IPv6. Also verify that the smarthost exception is not broader than intended.
Operational checklist
- Document the approved relay hostname, addresses, port and credentials owner.
- Deny unauthorized outbound TCP/25 at the provider or VPS boundary.
- Enable cPanel SMTP Restrictions when using cPanel/WHM.
- Configure all applications to submit through Exim with proper credentials.
- Add Exim ACL checks at the stage that matches your policy.
- Use
ratelimitwith a sender key that remains meaningful across connections. - Log accepted submissions, relay handoffs and policy rejections.
- Syntax-check and render panel-generated configuration before every reload.
- Test blocked direct delivery, approved submission, relay handoff and rate-limit behavior.
- Re-test after provider, panel or Exim updates because generated configuration and network policy can change.
Which control should you use?
| Goal | Primary control | Why it matters |
|---|---|---|
| Stop a compromised process from bypassing Exim | Provider/VPS egress deny for TCP/25 | Works even when the process never uses the MTA. |
| Prevent users and services from opening remote SMTP sessions | cPanel SMTP Restrictions or equivalent local policy | Limits remote SMTP access to Exim, Mailman and root in cPanel’s documented model. |
| Slow a legitimate-looking but abusive sender | Exim ACL with ratelimit |
Measures activity across multiple connections and provides a policy response. |
| Continue sending when port 25 is unavailable | Authenticated smarthost on 587 or 465 | Uses documented submission ports while retaining Exim as the controlled MTA. |
Frequently Asked Questions
Will blocking outbound port 25 stop applications from sending mail?
It will stop direct Internet delivery on that port. Applications can still send when they submit to local Exim, which then relays through the configured smarthost on authenticated port 587 or 465.
Does cPanel SMTP Restrictions replace an outbound firewall rule?
No. SMTP Restrictions prevents ordinary users and services from connecting directly to remote SMTP servers through the cPanel policy. A provider or VPS egress rule is still the control that blocks a process attempting to bypass Exim entirely.
Is the 100 messages per hour value a recommended Exim limit?
No. It is an illustrative ACL threshold. Set the limit from your application’s normal volume, relay policy and alerting needs; Exim does not publish one universal safe number.
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Why can an Exim ACL affect local scripts as well as network clients?
Exim ACLs govern SMTP commands, including submissions made by local processes through Exim’s -bs interface, so the same policy can cover both connection types when placed at the appropriate ACL stage.
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