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To set up Sendmail on an Ubuntu VPS, install sendmail, sendmail-cf, mailutils and the m4 macro processor; verify the VPS hostname; edit /etc/mail/sendmail.mc; regenerate /etc/mail/sendmail.cf; configure a smart host and SMTP credentials when direct delivery is unavailable; restrict relay access; restart Sendmail; and send a test message.
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This procedure is based on third-party Ubuntu and Linux guides rather than current official Ubuntu or Sendmail documentation. Package names, service behavior and provider policies can differ by Ubuntu release. Treat every command as a starting point and verify it on the release image and relay service you actually use.
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What you are configuring
Sendmail is the mail-transfer agent (MTA) that accepts a message from a local command or application and then hands it to the next SMTP server. On a VPS, there are two possible paths:
- Direct delivery: Sendmail connects to each recipient domain’s mail server. This requires the VPS and its network provider to permit outbound SMTP traffic and for receiving providers to accept mail from the server’s identity.
- Smart-host delivery: Sendmail hands all outbound mail to a designated SMTP relay. The relay performs the onward delivery. This is the usual approach when a VPS provider blocks or limits outbound port 25.
The important configuration rule is that sendmail.mc is the source file you edit. Sendmail actually reads the generated sendmail.cf, so changes in the macro file do nothing until you rebuild the configuration and restart the service.
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Do not expose a newly installed server as an unrestricted relay. The access database and authentication settings must limit who can submit mail through it.
Before installing: decisions and prerequisites
Choose direct delivery or a smart host
Ask the VPS provider whether outbound TCP port 25 is allowed. If it is blocked, throttled or filtered, choose an SMTP relay that accepts authenticated submissions, commonly on port 587. The relay hostname, port, authentication method and credentials must come from that relay service. A smart host is not a workaround for missing credentials: Sendmail still needs the relay’s required authentication settings.
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Check /etc/hosts before running Sendmail’s setup. The localhost entry and the VPS hostname should both be present. A minimal check is:
cat /etc/hostname
cat /etc/hosts
hostname --fqdnIf the hostname shown by the commands does not match the name you intend to use, correct the operating-system hostname and its corresponding /etc/hosts entry first. A mismatched identity can produce confusing configuration and delivery errors, so do not continue until the local names are consistent.
Have administrative access and relay details ready
- An account with
sudorights. - The relay hostname, such as
smtp.isp.com, if you will use a smart host. - The relay port and authentication requirements. The procedure below describes port 587 settings because one documented configuration uses that submission port.
- A test recipient address that you control.
Install the Sendmail packages
Install the MTA, its configuration macros and local mail utilities. The guides used for this procedure install sendmail, sendmail-cf and mailutils; one also requires m4 to regenerate the configuration.
sudo apt update
sudo apt install sendmail sendmail-cf mailutils m4Package names and dependency prompts can vary with the Ubuntu release. If the package manager reports that one of these names is unavailable, stop and check the package set for that release rather than substituting a similarly named package at random. The current release packaging was not verified by the sources used for this guide.
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After installation, do not make manual edits to the generated sendmail.cf as your primary configuration method. Put durable changes in /etc/mail/sendmail.mc and regenerate the compiled file.
Configure the macro file
Back up the existing files
Create a copy before changing a working image:
sudo cp /etc/mail/sendmail.mc /etc/mail/sendmail.mc.backup
sudo cp /etc/mail/sendmail.cf /etc/mail/sendmail.cf.backupIf you need to roll back, restore the macro backup, rebuild sendmail.cf, and restart Sendmail. Keeping the source and generated files paired prevents you from restoring one without the other.
Set a smart host when the VPS cannot deliver directly
Open the macro file:
sudo nano /etc/mail/sendmail.mcEnable the smart-host definition and point it at the relay supplied by your mail provider. The documented pattern is:
define(`SMART_HOST', `smtp.isp.com')dnlUse the exact hostname supplied by your relay. The older examples that use a provider-specific host are tied to older Ubuntu releases and provider policies; do not copy their provider details merely because the syntax looks familiar.
If you are intentionally delivering directly, leave the smart-host setting disabled and first verify that the VPS permits outbound SMTP. A smart host and direct delivery are different operating modes; mixing partial settings from both makes troubleshooting harder.
Configure port 587 submission
One documented port-587 configuration sets both the relay and ESMTP mailer arguments to TCP port 587:
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define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnlThe same configuration enables the authentication mechanisms and authentication options required by the relay, using TRUST_AUTH_MECH and confAUTH_OPTIONS. Their exact values depend on what the relay accepts, so copy the mechanism and option values specified by that service rather than inventing a value. Keep these definitions in sendmail.mc, not only in the generated file.
Add SMTP authentication safely
Create the authinfo source file
Sendmail reads relay credentials from /etc/mail/authinfo when the authentication feature is enabled in the macro file. Create that file with root-only access:
sudo touch /etc/mail/authinfo
sudo chmod 600 /etc/mail/authinfo
sudo nano /etc/mail/authinfoEnter the authentication record in the syntax required by your relay. A typical record identifies the relay host and supplies a username, password and authentication mechanism; use the relay’s documented values and mechanism. Do not paste credentials into sendmail.mc, a shell history or a file readable by ordinary users.
Build the hashed map
After saving the source file, build the database that Sendmail will query:
sudo makemap hash /etc/mail/authinfo < /etc/mail/authinfoThis creates the hash map alongside the source file. Re-run the command whenever the credentials or relay hostname changes. The authentication feature must also be enabled in sendmail.mc; creating the map alone does not make Sendmail use it.
Protect and rotate credentials
- Keep
/etc/mail/authinfoowned by root and readable only by the account that needs it. - Use a dedicated relay credential instead of a personal mailbox password where the relay supports that option.
- When a credential changes, edit the source file, rebuild the map, regenerate
sendmail.cf, then restart Sendmail. - Never include the password in a test command or a support request.
Restrict who may relay
Edit /etc/mail/access to list the hosts that are allowed to relay. Start with only the local systems and networks that genuinely submit mail. Do not add an entire public address range for convenience.
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sudo nano /etc/mail/accessThe access file is a source database, so rebuild its map after editing. The standard command used in the guides is:
sudo makemap hash /etc/mail/access < /etc/mail/accessRestricting relay access is separate from SMTP authentication. Authentication proves that a client knows a credential; the access database limits which hosts Sendmail will trust. Use both controls when your deployment accepts submissions from more than the local machine.
Regenerate sendmail.cf
With the macro file, authentication source and access file ready, regenerate the active configuration. There are two documented approaches:
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sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cfUse the make target
sudo make -C /etc/mailUse one method consistently after each macro-file change. The generated sendmail.cf should have a current timestamp after the command completes. If it does not, check the command’s error output and fix the macro syntax before restarting.
Start, restart and verify the service
Restart Sendmail so the daemon loads the regenerated configuration:
sudo systemctl restart sendmail
sudo systemctl status sendmailThe status command should show an active service. If it reports a failure, do not proceed to delivery testing. Recheck the last edits to sendmail.mc, the relay definitions, file permissions and the output from the m4 or make command. Restore the backed-up macro file if necessary, rebuild sendmail.cf, and restart again.
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Send a controlled test message
Use the local sendmail command to submit a small message and request verbose output:
echo "test message" | sendmail -v your@addressReplace your@address with an address you control. The -v option prints the SMTP conversation or delivery result, which helps distinguish local submission from relay failure. Confirm both that the message arrives and that it is not placed in the recipient’s spam folder.
Run a second test after any change to the smart host, authentication map, access map or generated configuration. A successful local command only proves that Sendmail accepted the message; the recipient’s mailbox and spam folder provide the final delivery check.
Troubleshoot the common failure paths
The service will not start
- Check that
/etc/mail/sendmail.mccontains valid macro syntax and that the m4 or make command completed without errors. - Confirm that
/etc/hostsincludes localhost and the server hostname. - Rebuild
sendmail.cfafter every macro edit; restarting without rebuilding leaves the old configuration active. - Restore the backup files if the last change introduced the failure, then rebuild and restart.
The message is accepted but never leaves the VPS
- Verify whether the VPS provider blocks outbound port 25. If it does, configure a smart host and the relay’s submission port instead of attempting direct delivery.
- Check the smart-host hostname for spelling and ensure the generated
sendmail.cfcontains the new setting. - For port 587, confirm that the relay and ESMTP mailer arguments were enabled and that the authentication feature is active.
Authentication fails
- Confirm that
/etc/mail/authinfocontains the relay host named in the smart-host setting. - Rebuild the hash map with
makemapafter changing the file. - Check that the authentication mechanism and
TRUST_AUTH_MECH/confAUTH_OPTIONSvalues match the relay’s requirements. - Regenerate
sendmail.cfand restart; changing only the source file does not reload it.
The relay rejects the destination or performs an unexpected lookup
Some older examples wrap the relay hostname in square brackets, such as [smtp.gmail.com], to prevent an MX lookup and force a direct connection to that host. That example is tied to an older tutorial and provider details; use bracketed notation only when your relay’s current instructions require it. Do not assume that an old provider-specific recipe remains valid.
Mail is delivered to spam
The basic test command confirms submission and delivery status, not inbox placement. Check the recipient’s spam folder and the relay’s own delivery response. Because provider reputation and DNS requirements were not established by the available guides, do not infer that a successful Sendmail setup guarantees inbox placement.
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If unrelated external hosts can submit mail through the VPS, stop the service or block external access while correcting the configuration. Review /etc/mail/access, rebuild its map, regenerate sendmail.cf, and restart. Permit only the local hosts or authenticated submission paths that your application needs.
A repeatable change checklist
- Record the current Ubuntu release, hostname and relay settings.
- Back up
sendmail.mcandsendmail.cf. - Edit
sendmail.mcfor the smart host, port and authentication features. - Edit
/etc/mail/authinfo, protect it, and rebuild its hash map. - Edit
/etc/mail/accessand rebuild its hash map. - Regenerate
sendmail.cfwith m4 ormake -C /etc/mail. - Restart Sendmail and check
systemctl status sendmail. - Send a verbose test message and verify the destination mailbox and spam folder.
- Retest after every relay, credential, access or macro change.
What this setup does—and does not—establish
A working installation proves that the VPS can run Sendmail, accept a local message and, when configured, submit it to the selected relay. It does not establish that every Ubuntu release uses identical package defaults, that every VPS provider permits direct port-25 delivery, or that a recipient will place the message in its inbox. The available procedural material was third-party guidance, and no current official Ubuntu or Sendmail documentation was located for verification. Treat release-specific behavior and relay policies as items to confirm with the image and service you operate.
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