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To run Sendmail on an Ubuntu VPS, install sendmail, sendmail-cf, mailutils, and the m4 macro processor; verify the server hostname; configure /etc/mail/sendmail.mc; regenerate sendmail.cf; restart the service; and send a test message. If your VPS provider filters outbound SMTP on port 25, configure a smart host (SMTP relay), add its credentials through an authinfo map, and use port 587 when required. Restrict relay access before sending production mail so the machine cannot become an open relay.
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What you need before installing Sendmail
Sendmail is a mail transfer agent (MTA): it accepts messages from local programs and either delivers them directly to recipient mail servers or hands them to another SMTP server. On a VPS, the second arrangement is often more practical because hosting networks commonly limit outbound connections to TCP port 25. A smart host (also called a relay) accepts your server’s messages and performs onward delivery.
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The procedure below reflects commonly documented Ubuntu setups, but Sendmail defaults and package behavior can change between Ubuntu releases. Confirm that the packages and service names shown here exist in the release installed on your VPS, and use your relay provider’s current connection and authentication requirements.
- Root or
sudoaccess to the VPS. - A hostname for the VPS, with matching local name resolution.
- An SMTP relay hostname and credentials if direct delivery on port 25 is unavailable or disallowed.
- A destination mailbox you control for testing.
Do not expose a newly installed MTA to the internet until relay permissions are restricted and a test confirms that only intended senders can submit mail.
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Install the Sendmail packages
Update the package index and install the MTA, its configuration macros, local mail utilities, and the macro processor used to build the final configuration:
sudo apt update
sudo apt install sendmail sendmail-cf mailutils m4Some guides list only the first three packages and add m4 separately. Installing all four avoids a failed configuration rebuild when the macro processor is absent. If apt reports that a package is unavailable, stop and check the Ubuntu release and enabled repositories rather than substituting a similarly named package.
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Verify the VPS hostname and /etc/hosts
Sendmail uses the system name when it creates message headers and identifies itself during SMTP conversations. Before editing Sendmail, inspect the host file:
cat /etc/hosts
hostnameThe localhost entry and the VPS hostname should both be present. If the hostname is missing or resolves inconsistently, correct the host file using the naming scheme supplied by your VPS provider, then open a new shell or restart the relevant service as appropriate. A malformed host file can cause setup scripts to pause, produce an unexpected mail name, or fail when Sendmail tries to determine its local identity.
Use one consistent fully qualified name throughout your operating-system and Sendmail configuration. Avoid changing the hostname midway through troubleshooting; otherwise, a message may appear to originate from a different name than the one you tested.
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Understand Sendmail’s configuration files
Sendmail normally uses two related files:
/etc/mail/sendmail.mcis the human-edited macro source. It contains readable directives and feature definitions./etc/mail/sendmail.cfis the generated configuration consumed by Sendmail.
Edit the .mc file, not the generated .cf file. Direct edits to sendmail.cf are difficult to maintain and will be lost when the file is regenerated. Keep a backup before changing the macro source:
sudo cp /etc/mail/sendmail.mc /etc/mail/sendmail.mc.backupAfter every meaningful edit, regenerate sendmail.cf. Two commonly documented methods are:
sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
sudo make -C /etc/mailUse one method consistently. The first command may fail at the shell redirection step on systems where your unprivileged shell cannot write to /etc/mail; in that case, run the redirection through a privileged shell:
sudo sh -c 'm4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf'Read the command output carefully. A macro typo can produce a configuration that builds but does not behave as intended, so a successful command is not a substitute for a delivery test.
Choose direct delivery or a smart host
When direct delivery is appropriate
With direct delivery, Sendmail looks up the recipient domain’s mail exchanger and connects to that server, normally over SMTP port 25. This avoids a third-party relay, but it depends on your VPS network allowing outbound port 25 and on recipient systems accepting mail from your VPS address.
Why VPS users commonly need a relay
Many VPS networks restrict or block outbound port 25 to reduce abuse. A timeout or connection refusal on port 25 can therefore be a network policy issue rather than a Sendmail syntax error. A smart host lets your server submit mail to an SMTP relay using the port and authentication method that relay accepts.
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Set the smart-host macro
Open the macro file:
sudo nano /etc/mail/sendmail.mcFind the existing SMART_HOST definition, or add one in the section where other define directives appear. Replace the example relay name with the hostname supplied by your relay service:
define('SMART_HOST', 'smtp.isp.com')dnlMacro quoting can differ between packaged examples. Preserve the quoting style already used in your file and ensure the macro name is exactly the one expected by that file. The relay hostname is not necessarily the same as your VPS hostname or the recipient’s domain.
Use brackets only when the relay requires them
Some older Ubuntu tutorials write the relay as a bracketed host such as [smtp.gmail.com]. Brackets tell Sendmail to use that host literally instead of performing an MX lookup. This can be useful for a specific relay configuration, but provider details in older tutorials may no longer apply. Use brackets only when your relay’s current instructions require literal-host behavior; otherwise, use the relay hostname normally.
Configure SMTP authentication with authinfo
A relay that requires authentication needs credentials in Sendmail’s authentication map. Create or edit:
sudo nano /etc/mail/authinfoUse the exact record format, username, password representation, and authentication mechanism specified by your relay. Do not paste credentials into a publicly readable file or commit this file to source control. The authinfo file is an input database; Sendmail reads the generated hash map, not just the plain-text source.
Build the hash database with:
sudo makemap hash /etc/mail/authinfo < /etc/mail/authinfoThis creates the map that Sendmail looks up at delivery time. Re-run the command whenever the credentials or relay record changes. If the command reports a parse error, inspect quotation marks, whitespace, and the record syntax required by your relay before restarting Sendmail.
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The authentication feature must also be enabled in sendmail.mc. Enable the feature in the same macro source file where you define the smart host, then regenerate sendmail.cf. Merely creating authinfo does not activate authentication.
After building the map, limit access to the credential files according to your server’s security policy. A leaked relay password can be used to send mail through the relay even if your Sendmail access rules are correct.
Use port 587 when the relay expects message submission
Port 587 is commonly used for authenticated message submission. If your relay instructs you to submit there, configure the Sendmail mailer arguments in sendmail.mc rather than assuming the default port:
define('RELAY_MAILER_ARGS', 'TCP $h 587')dnl
define('ESMTP_MAILER_ARGS', 'TCP $h 587')dnlThe same setup may require the authentication trust mechanism and authentication options to be enabled:
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TRUST_AUTH_MECH
confAUTH_OPTIONSUse the exact directives and values supplied for your relay. These settings are not interchangeable with a relay that expects unauthenticated port 25 delivery. After changing them, regenerate the configuration and restart the service.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Restrict which hosts may relay
Relay permission is separate from the smart-host destination. The smart host tells Sendmail where to forward mail; the access database controls which connecting hosts are allowed to use your server as a relay.
Edit the access source file:
sudo nano /etc/mail/accessList only the local hosts, networks, or authenticated senders that should be allowed to relay, following the access-rule syntax already present in your file. Do not add a broad wildcard merely to make a test succeed. An unrestricted rule can turn the VPS into an open relay, allowing outsiders to send through your server and damaging its reputation.
Rebuild the access database after every change:
sudo makemap hash /etc/mail/access < /etc/mail/accessKeep the source file and generated map synchronized. If you edit /etc/mail/access but do not run makemap, Sendmail may continue using the previous policy.
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Regenerate the configuration and restart Sendmail
Once the hostname, smart host, authentication, port, and access rules are ready, rebuild the configuration:
sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cfAlternatively:
sudo make -C /etc/mailRestart the service and immediately check its state:
sudo systemctl restart sendmail
sudo systemctl status sendmailA running service only proves that Sendmail accepted the configuration syntax. It does not prove that the relay is reachable, credentials work, or recipient systems will accept the message.
Send a test message and interpret the result
Send a small test message to an address you can inspect:
echo "test message" | sendmail -v your@addressThe -v option prints the SMTP conversation and delivery path. Save the output while troubleshooting, but remove passwords or other sensitive values before sharing logs.
Check all of the following:
- Sendmail connects to the intended relay rather than attempting an unreachable direct delivery.
- The connection uses the port required by the relay, such as 587 for authenticated submission.
- The relay accepts the authentication credentials without an authentication or authorization error.
- The final SMTP response indicates acceptance or successful hand-off.
- The message arrives in the destination mailbox, including its spam or junk folder.
A successful hand-off means the next mail server accepted responsibility; it does not guarantee placement in the inbox. Keep the test recipient, timestamp, verbose output, and relay response together so you can distinguish a local configuration problem from downstream filtering.
Troubleshoot the most common failures
| Symptom | Likely area | What to check |
|---|---|---|
sendmail is not found |
Packages | Confirm that sendmail and mailutils installed successfully; install the missing package with apt. |
| The service will not start | Generated configuration | Rebuild from sendmail.mc, read the error output, and undo the last macro change. Check for unmatched quotes or an invalid directive. |
| Port 25 times out | VPS network policy or routing | Use the relay supplied for your VPS and configure its required submission port instead of repeatedly retrying direct delivery. |
| Connection reaches the relay but authentication fails | authinfo map or feature settings |
Verify the relay hostname, credential record, authentication feature, and regenerated hash map. Re-run makemap after correcting the source. |
| Changes have no effect | Stale generated files | Regenerate sendmail.cf and the relevant map databases, then restart Sendmail. |
| The relay accepts mail but it never appears | Recipient filtering or spam placement | Check the spam folder and the relay’s delivery status. A local “accepted” response is not an inbox guarantee. |
| Unexpected outside mail is accepted | Relay access policy | Inspect /etc/mail/access, remove broad relay permissions, rebuild its map, and restart the service. |
| The message identifies the wrong server | Hostname or host file | Recheck the VPS hostname and the localhost/hostname entries in /etc/hosts, then regenerate the Sendmail configuration. |
A repeatable deployment checklist
- Confirm the Ubuntu release, obtain
sudoaccess, and choose the hostname. - Verify that
/etc/hostscontains valid localhost and VPS-hostname entries. - Install
sendmail,sendmail-cf,mailutils, andm4. - Back up
/etc/mail/sendmail.mc. - Decide whether direct port 25 delivery is permitted; if not, obtain a smart-host relay.
- Set the smart-host macro and, when required, the relay’s port-587 mailer arguments.
- Create the relay’s
authinfoentry, enable authentication, and runmakemap. - Restrict relay clients in
/etc/mail/accessand rebuild that map. - Regenerate
sendmail.cfwithm4ormake -C /etc/mail. - Restart Sendmail and verify its service status.
- Send a verbose test message and check both acceptance output and the recipient’s spam folder.
Repeat the regeneration and map-building steps whenever you change the macro file, authentication records, or access policy. Keep a dated backup of the last known-good configuration so you can restore service without guessing which directive changed.
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