To stop Sendmail accepting inbound SMTP on TCP port 25, remove or disable the daemon socket that binds the SMTP service, then regenerate the configuration, restart Sendmail, and verify both IPv4 and IPv6 listeners. On Oracle Linux 7, the documented switch is DAEMON=no in /etc/sysconfig/sendmail. On M4-managed installations, remove the MTA DAEMON_OPTIONS declaration from the .mc source. If local applications still need to submit mail, retain a loopback-only MSA on port 587 rather than exposing port 25.
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What disabling port 25 actually changes
Sendmail opens an incoming socket when it runs in daemon mode. The daemon normally uses SMTP port 25, and each DAEMON_OPTIONS occurrence can create another listening socket. The Port, Addr, and Family fields determine the port, bind address, and address family.
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Removing the MTA socket prevents remote SMTP clients from connecting to this host on port 25. It does not, by itself, remove every way to submit or deliver mail. Sendmail can still be used for local submission, queue processing, and outbound delivery, depending on the remaining configuration and how the service is started. The -bd daemon mode is the part associated with accepting connections on socket 25; message delivery and queue operation are separate modes.
There are two common end states:
- No incoming Sendmail sockets: remove the MTA listener and any MSA listener you do not need.
- Local submission only: remove the public MTA listener, disable the automatically generated MSA if necessary, and add an MSA on port 587 bound only to
127.0.0.1,::1, or both.
Identify how your distribution manages Sendmail
Do not apply the Oracle setting to every Linux distribution. First determine whether the package exposes a distribution startup file or builds sendmail.cf from an M4 source file.
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Oracle Linux 7 startup configuration
Oracle Linux 7 documents a setting in /etc/sysconfig/sendmail. In that file, DAEMON=no disables Sendmail daemon startup, which prevents it from listening for incoming mail on port 25. This is a distribution-specific control, so inspect the installed file and the package documentation before changing it.
M4-managed configuration
Many installations maintain an editable .mc file and generate the active sendmail.cf from it. In this model, the .cf file is generated output. Editing only that output can be lost the next time the package rebuilds the configuration. Make the durable change in the .mc source, then regenerate and install the resulting sendmail.cf using the procedure supplied by your distribution.
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Option 1: disable the daemon on Oracle Linux 7
- Open
/etc/sysconfig/sendmailwith an administrator account. - Set the daemon switch to exactly
DAEMON=no. Preserve unrelated variables in the file. - Save the file and restart Sendmail using the service-management command used by your Oracle Linux installation.
- After the restart completes, verify the listening sockets with the checks in the verification section below.
This switch disables daemon startup rather than selectively changing one generated DAEMON_OPTIONS line. It is appropriate when the host should not accept incoming SMTP at all. If local software must submit mail through an MSA on port 587, use the M4 approach instead, or confirm that your package provides a separate supported MSA configuration.
Option 2: remove the MTA listener in an M4 configuration
Remove the standard SMTP declaration
Sendmail’s configuration documentation identifies the default MTA declaration as:
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DAEMON_OPTIONS(`Port=smtp, Name=MTA')Delete or comment out that MTA declaration in the .mc source when no inbound SMTP socket should exist. The documentation states that omitting the first MTA DAEMON_OPTIONS line leaves no listener on the standard SMTP port. Remove every equivalent public MTA declaration, not just the first line you notice; each remaining declaration can create another socket.
Do not confuse the MTA declaration with an MSA declaration. An MTA normally receives server-to-server mail on port 25. An MSA normally accepts mail submissions from trusted local clients on port 587. They are separate listener definitions.
Prevent an unwanted automatically generated MSA
If you want no Sendmail listener at all, add this feature to the .mc source:
FEATURE(`no_default_msa')FEATURE(`no_default_msa') prevents automatic generation of the default MSA entry. Use it when removing the MTA line is not enough because the generated configuration would otherwise still create an MSA socket.
Keep local submission on port 587
For a host that needs local applications to submit messages but must not expose SMTP publicly, combine FEATURE(`no_default_msa') with explicit loopback-only MSA declarations. A typical pattern is:
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FEATURE(`no_default_msa')
DAEMON_OPTIONS(`Port=587,Addr=127.0.0.1, Name=MSA')
DAEMON_OPTIONS(`Port=587,Addr=::1, Name=MSA')Use only the address families your system needs. The IPv4 entry accepts connections through the IPv4 loopback address; the IPv6 entry accepts connections through the IPv6 loopback address. Neither binds a public interface. If applications use only one family, retaining only that loopback declaration reduces unnecessary sockets.
The exact option spelling and surrounding lines must match the syntax already used in your package’s .mc file. Keep authentication, access-control, and submission policy settings that your local clients require; changing the bind address alone does not create a complete application policy.
Regenerate, install, and restart safely
- Back up the current
.mcsource and generatedsendmail.cfso you can restore a known-good configuration. - Edit the source file: set
DAEMON=nofor the Oracle Linux 7 startup-file method, or remove the MTA declaration and adjust MSA features in the M4 method. - Build a new
sendmail.cffrom the edited.mcfile with the M4 build command documented by your installed Sendmail package. - Install the generated file in the location loaded by the service. Do not assume that the directory used by one distribution is correct for another.
- Restart Sendmail through the package’s service manager. A restart is required; editing a source file does not change sockets held by an already running daemon.
- Check the service status and logs for a configuration or syntax error before testing from a remote system.
If the generated file fails to build, restore the backup and correct the M4 syntax before restarting. A partially written or invalid configuration can prevent Sendmail from starting, while an old running process may continue to hold port 25 until it is stopped.
Verify that port 25 is closed on every address family
Inspect all TCP listeners
Run:
ss -ltnpReview the local-address column for any listener using :25. A public IPv4 listener commonly appears as 0.0.0.0:25 or as a specific non-loopback IPv4 address. A public IPv6 listener appears as [::]:25 or a specific non-loopback IPv6 address. Either one means the host can still accept connections on port 25 through that address family.
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Confirm the intended submission sockets
If you retained local submission, the only expected Sendmail listener should be the loopback address or addresses you deliberately configured, such as 127.0.0.1:587 and/or [::1]:587. A wildcard address on port 587 is not loopback-only and should be treated as a separate exposure that requires an intentional decision.
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Check the owning process
The process column from ss -ltnp should identify the Sendmail daemon for any remaining Sendmail socket. If another process owns port 25, changing Sendmail will not remove that listener. Investigate the owning service separately before declaring the port closed.
Test from outside the host when remote access matters
A local socket listing tells you what the kernel has bound, while an external connection test confirms what a remote network can reach. Test both an IPv4 route and an IPv6 route when the host has both. A firewall can block a reachable service, but a firewall rule alone does not prove that Sendmail stopped listening; the authoritative local check is the absence of a port-25 socket.
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Blocking port 25 only in a firewall
A firewall rule can prevent selected traffic while Sendmail continues to listen. That leaves an unnecessary daemon socket, may permit traffic from an allowed interface, and can obscure the actual service state. Remove the MTA listener in Sendmail’s configuration first, then use firewall policy as an additional network control.
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Generated files can be replaced during a package update or a later M4 rebuild. If the MTA line returns after regeneration, the source .mc file still declares it. Make the source change permanent and treat the generated file as build output.
Stopping the service without changing startup configuration
A one-time stop removes the current process but does not prevent the service from returning after a reboot or package action. Use the supported startup switch or source configuration, then restart and verify the result.
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Removing the MTA but forgetting IPv6
Administrators often check only 0.0.0.0:25. A separate IPv6 declaration can leave [::]:25 active, and Sendmail creates an incoming socket for each daemon declaration. Always inspect both address families.
Removing every MSA entry when local mail is still needed
FEATURE(`no_default_msa') is correct when no MSA should exist, but it also removes the automatically generated MSA. If local programs submit through port 587, add explicit loopback-bound MSA declarations after disabling the default. Otherwise those programs may fail even though remote SMTP is correctly blocked.
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Assuming a successful restart proves the port is closed
Sendmail can restart successfully while a different daemon declaration still binds port 25, or while another process owns the port. Use ss -ltnp after every configuration change and identify all remaining listeners.
Choose the approach that matches the requirement
| Approach | Configuration point | Port-25 result | Local submission | Persistence considerations |
|---|---|---|---|---|
| Oracle Linux 7 daemon switch | /etc/sysconfig/sendmail with DAEMON=no |
Sendmail daemon does not start to accept incoming SMTP | Not established by this switch; verify the package’s separate submission behavior | Uses the distribution’s startup configuration; inspect it after package changes |
| M4 source without MTA declaration | Remove the MTA DAEMON_OPTIONS line from the .mc source |
No standard SMTP listener is generated from that declaration | Only if an MSA remains or is explicitly added | Survives regeneration when the source file is maintained |
| M4 source with loopback MSA | Disable the default MSA and add port-587 entries bound to 127.0.0.1 and/or ::1 |
No public port-25 listener | Yes, for clients using the configured loopback address | Regenerate and reinstall from the maintained .mc source |
| Firewall-only block | Network firewall rules | May block traffic, but Sendmail can still listen | Depends on the firewall rule and listener configuration | Does not change Sendmail’s daemon configuration |
Operational checklist
- Identify whether the host uses Oracle Linux 7’s startup file or an M4-managed configuration.
- Back up the active source and generated configuration.
- Disable the daemon with
DAEMON=no, or remove every public MTA declaration from the.mcsource. - Decide explicitly whether an MSA is required. Add only loopback-bound port-587 sockets when local submission must remain.
- Regenerate and install
sendmail.cfusing the installed package’s procedure. - Restart Sendmail and inspect status or logs for errors.
- Use
ss -ltnpto confirm that neither public IPv4 nor public IPv6 has a:25listener. - Confirm that any remaining
:587listener is limited to the intended loopback address. - Repeat the verification after upgrades or configuration rebuilds, because generated files and service defaults can change.
Final answer
The durable fix is to change Sendmail’s daemon configuration, not merely to filter port 25. On Oracle Linux 7, set DAEMON=no in /etc/sysconfig/sendmail. On an M4-managed system, remove the MTA DAEMON_OPTIONS declaration, add FEATURE(`no_default_msa') when no MSA is wanted, or replace the default with explicit 127.0.0.1:587 and/or ::1:587 entries for local submission. After rebuilding and restarting, verify that ss -ltnp shows no public :25 listener in either address family.
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