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To set up Sendmail on an Ubuntu VPS, install sendmail, sendmail-cf, mailutils and the m4 macro processor; verify the server hostname; configure /etc/mail/sendmail.mc; rebuild sendmail.cf; add relay authentication when your provider requires it; restrict which hosts may relay; restart Sendmail; and send a test message. A smart host on port 587 is usually the practical path when a VPS provider blocks outbound port 25.
What this setup does—and what is uncertain
This guide configures Sendmail as an outbound SMTP client on an Ubuntu VPS. It does not turn the VPS into a full mail-hosting platform with mailboxes, IMAP, spam filtering or webmail. Your application hands a message to the local Sendmail process, which then delivers directly or forwards it to a smart host (an external SMTP relay).
The available Ubuntu and Sendmail procedures for this topic come from third-party guides rather than current official Ubuntu or Sendmail documentation. Package names, service defaults and macro behavior can differ between Ubuntu releases, so check the package candidate and the relay provider’s current requirements before applying a production configuration. The commands below show the established workflow and identify the places where your release or provider may differ.
Prerequisites and decisions
- An Ubuntu VPS with
sudoaccess and a resolvable hostname. - An address at which you can receive the test message.
- Either a relay service (recommended for many VPS deployments) or a provider that explicitly permits direct SMTP delivery from your server.
- If the relay requires login, its hostname, port, username, password or token, and permitted authentication mechanisms.
Before editing anything, decide whether Sendmail should connect directly to recipient mail servers or use a smart host. Direct delivery normally uses SMTP port 25. VPS networks frequently restrict outbound port 25, in which case Sendmail will queue or fail messages until it is pointed at a relay that accepts authenticated submission, commonly on port 587. The relay’s hostname and credentials are provider-specific; do not copy credentials or mechanism values from an unrelated tutorial.
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Install the Sendmail packages
- Refresh APT metadata:
sudo apt update - Install the MTA, configuration macros and command-line mail tools:
sudo apt install sendmail sendmail-cf mailutils m4Some guides list
m4separately because it is the processor used to turn the human-edited macro file into Sendmail’s generated configuration. If APT reports that a package name is unavailable on your Ubuntu release, inspect the package list withapt-cache policy sendmail sendmail-cf mailutils m4and use the package names supplied by that release rather than forcing an old repository. - During installation, a configuration dialog may appear. Complete it with the server’s real hostname and domain information, but plan to review the generated files manually before sending production mail.
Verify the hostname and /etc/hosts
Sendmail uses the local hostname when constructing queue and envelope information. A malformed hostname can produce warnings, rejected messages or confusing headers. Confirm the values first:
hostname
hostname -f
cat /etc/hostname
cat /etc/hostsThe localhost entries and the VPS hostname should be present in /etc/hosts before you run Sendmail’s setup. The exact address depends on your VPS network, so do not replace an existing mapping with a copied example. A typical file contains a localhost line and a separate line mapping the server’s own address to its fully qualified hostname and short name. If your provider documents a hostname format, use that format consistently in the system hostname, DNS and Sendmail configuration.
Know which Sendmail files you are changing
| Path | Purpose | How to handle it |
|---|---|---|
/etc/mail/sendmail.mc |
Macro configuration that you edit | Keep your intended settings here; this is the source file for regeneration. |
/etc/mail/sendmail.cf |
Generated Sendmail configuration | Do not make one-off edits here; regenerate it after changing the .mc file. |
/etc/mail/access |
Hosts and patterns allowed to relay | Enter only trusted sources, then build its database. |
/etc/mail/authinfo |
Relay authentication records | Protect the file and generate the hash map used by Sendmail. |
/etc/mail/*.db |
Compiled maps, including access and authinfo | Rebuild the relevant map whenever its source file changes. |
Back up the files before editing:
sudo cp -a /etc/mail/sendmail.mc /etc/mail/sendmail.mc.bak
sudo cp -a /etc/mail/access /etc/mail/access.bak 2>/dev/null || true
sudo cp -a /etc/mail/authinfo /etc/mail/authinfo.bak 2>/dev/null || trueConfigure a smart host
Open the macro file:
sudo editor /etc/mail/sendmail.mcEnable the smart-host setting and replace the example hostname with the SMTP relay supplied by your provider. The conventional m4 form is:
define(`SMART_HOST', `smtp.relay.example')dnlOne Oracle Linux example writes the macro name as SMART_host. That spelling is shown in older material and may not match the macro set shipped by your Ubuntu package. Use the macro already present in your file or the spelling documented for your installed Sendmail configuration; do not assume that a line copied from an older distribution will be accepted unchanged.
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Some older Ubuntu 16.04/18.04 instructions put the relay in brackets, for example [smtp.gmail.com], to prevent an MX lookup and force a connection to the named SMTP host. The provider details in that tutorial are dated. Use bracketed syntax only when your relay provider or installed configuration explicitly requires it.
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Add SMTP authentication safely
Create the authentication source file with permissions that prevent ordinary users from reading the credential:
sudo install -o root -g root -m 600 /dev/null /etc/mail/authinfo
sudo editor /etc/mail/authinfoA common authinfo record has this shape (replace every value with the relay’s required values):
AuthInfo:smtp.relay.example "U:relay-user" "P:replace-with-password" "M:PLAIN"The username, password or token, and mechanism string are examples of fields, not universal values. Your provider may require LOGIN, PLAIN, another mechanism, or a token in place of a password. Keep the record’s host name aligned with the host used by the smart-host setting.
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Enable the authinfo feature in sendmail.mc using the syntax supplied by your package. A commonly deployed form is:
FEATURE(`authinfo', `hash -o /etc/mail/authinfo.db')dnlIf your installed macro file already contains an authinfo feature, edit that line instead of adding a duplicate. Then generate the database:
sudo makemap hash /etc/mail/authinfo < /etc/mail/authinfoThe command creates or refreshes /etc/mail/authinfo.db. Re-run it whenever the source credentials change. Check ownership and permissions on both the source and database files, and avoid putting the password in shell history or a publicly readable backup.
Use port 587 when the relay requires submission
Port 587 is the submission port used by many authenticated relays. A documented Sendmail example sets both relay mailer argument macros to TCP port 587:
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define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnlThe same configuration family enables TRUST_AUTH_MECH and confAUTH_OPTIONS. Add those directives only with the authentication mechanisms and options required by your relay. They are not interchangeable with the username and password, and an overly broad trust setting can allow unauthenticated clients to submit mail.
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If the provider requires STARTTLS, certificate validation or a particular authentication mechanism, follow its current connection requirements. A port change alone does not enable encryption or authentication.
Rebuild sendmail.cf after every macro change
Sendmail reads the generated .cf file, not the comments and macros you just edited. Rebuild it with either of these supported workflows:
sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cfor:
sudo make -C /etc/mailUse one method consistently with the layout provided by your Ubuntu package. If the command reports an m4 syntax error, inspect the last line you changed for mismatched backticks, apostrophes or parentheses. Do not start Sendmail with a partially generated file; restore the backup, correct the macro and regenerate it cleanly.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Restrict relaying to trusted hosts
An SMTP server that relays for arbitrary Internet clients can be abused and quickly acquire a poor reputation. Edit the access map:
sudo editor /etc/mail/accessList only the local machine, explicitly trusted application hosts and any other source that must submit mail. The exact left-hand key depends on whether you identify a source by hostname, address or network; use the format already documented in your installed file. A typical policy uses the RELAY action for approved entries and leaves all other sources untrusted. Do not add an entire public network or a wildcard merely to make a test pass.
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Compile the map after saving:
sudo makemap hash /etc/mail/access < /etc/mail/accessIf Sendmail’s macro file does not already enable the access feature, enable the feature required by your package before rebuilding sendmail.cf. Changing access without rebuilding its database leaves Sendmail using the old policy.
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- Regenerate
sendmail.cfafter all macro edits and rebuild both map databases. - Restart Sendmail:
sudo systemctl restart sendmail - Check the unit state and recent diagnostics:
sudo systemctl status sendmail sudo journalctl -u sendmail -e - Submit a message through the local Sendmail command, replacing the address with one you control:
echo "test message from $(hostname -f)" | sendmail -v recipient@example.netThe
-voption prints the SMTP conversation available to the local client. A successful handoff means Sendmail accepted the message; it is not proof that the recipient accepted it or placed it in the inbox. - Check the recipient’s inbox and spam folder. If it does not arrive, use the timestamp and queue identifier in the Sendmail log to distinguish local configuration errors from a remote rejection.
Troubleshoot by symptom
Connection times out or port 25 is refused
This commonly indicates an outbound port-25 policy on the VPS network or a firewall rule. Ask the provider whether direct SMTP is blocked, then configure the provider’s smart host and authenticated submission port (often 587). Do not repeatedly retry direct delivery while a network policy is rejecting it; messages will accumulate in the queue.
Authentication fails
Confirm that the hostname in authinfo exactly matches the relay host used by Sendmail, that the authinfo feature is enabled, and that /etc/mail/authinfo.db was rebuilt after the last edit. Check the provider’s required mechanism and whether the account expects a token rather than its normal web password. Rebuild sendmail.cf after changing any related macro.
Messages remain queued
Inspect the service log and queue diagnostics for the specific SMTP response. A queued message can result from a temporary remote failure, DNS or network connectivity, an unavailable relay, or a rejected authentication attempt. Correct the underlying error, then allow Sendmail’s normal retry behavior or process the queue using the queue command supported by your installed release.
Sendmail starts but ignores your setting
Verify that you edited /etc/mail/sendmail.mc, not only sendmail.cf; regenerate the .cf file; rebuild the relevant map; and restart the service. A syntactically valid old configuration can continue running even when a new macro edit was never compiled.
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The test command succeeds but no mail arrives
Read the verbose output and the Sendmail log for the final remote response. Confirm the recipient address, inspect spam or quarantine folders, and check whether the relay accepted the message for later delivery. Local command success only establishes that Sendmail accepted the message from the shell.
An older tutorial gives different directives
Sendmail tutorials written for Ubuntu 16.04 or 18.04 may contain provider-specific hostnames, bracketed relay syntax or macro names that do not match a current package. Treat those values as historical examples. Keep the general sequence—edit .mc, generate .cf, compile maps, restart, test—but validate each directive against the relay and the Sendmail files installed on your VPS.
Operational checklist before production use
- Hostname and
/etc/hostsentries are internally consistent. - The smart-host hostname and submission port match the relay account.
- Credentials are stored in
/etc/mail/authinfowith restrictive permissions, and the hash map is current. - Only required hosts receive the
RELAYaction in/etc/mail/access. sendmail.cfwas regenerated fromsendmail.mcafter the final edit.- The Sendmail service restarts cleanly and its status shows no startup error.
- A test message reaches a monitored mailbox, and you have recorded the relevant log response.
- You know how to revoke or rotate the relay credential if the VPS is compromised.
Once these checks pass, keep the generated configuration and map files backed up alongside the source files, document the relay hostname and port, and monitor Sendmail’s logs and queue. Revisit the setup after an Ubuntu upgrade or a relay-provider policy change, because package defaults and accepted authentication methods are not guaranteed to remain identical.
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