The quickest reliable method is to install Ubuntu’s supported mail transfer agent (Postfix) with Mailutils, then pipe a message into mail:
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sudo apt update
sudo apt install postfix mailutils
printf '%sn' 'Body text from the server' | mail -s 'Ubuntu mail test' [email protected]That command submits the message to the local Postfix queue. It does not, by itself, prove that the recipient’s provider accepted the message or placed it in the inbox. For a server that should relay through a managed SMTP service, configure authenticated SMTP with TLS or use a direct SMTP client such as SendEmail or Mailutils.
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A terminal command is normally a mail user agent (MUA) or a thin client. It hands the message to a mail transfer agent (MTA), which queues it, resolves the destination, opens an SMTP connection, retries temporary failures and records diagnostics. A mail delivery agent (MDA) can then place locally addressed mail in a mailbox that users read through POP3 or IMAP.
Ubuntu documents this chain and states: “On Ubuntu, Postfix is the default supported MTA.” Postfix is therefore the natural local hand-off point for cron jobs, monitoring scripts and system notifications. The command’s exit status generally tells you whether the local process or SMTP server accepted the submission; final delivery depends on DNS, authentication, relay policy, sender reputation and the recipient’s filtering.
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Choose the sending path
| Method | Best fit | Queue and retry behavior | Credential and TLS considerations | Diagnostics |
|---|---|---|---|---|
Postfix plus mail/Mailutils |
Cron and system scripts on a host that already has a controlled MTA | Postfix keeps a queue and retries deferred deliveries | Credentials and TLS are centralized in Postfix configuration; protect the files | Postfix queue, journal and mail logs |
| SendEmail | A small one-off or scripted submission to a specified SMTP endpoint | The SMTP server, rather than the client, normally provides durable queueing | Pass an authenticated SMTP account and use the relay’s TLS settings; avoid exposing secrets in history or process listings | Use -v and inspect the relay response |
| Mailutils SMTP client | SMTP URL parameters or interactive protocol diagnostics | Depends on the SMTP server when connecting directly | Supports SMTP URLs, authentication mechanisms and STARTTLS; do not leave passwords in shell history | Interactive ehlo, starttls and auth steps show capabilities |
For a send-only Ubuntu server, local Postfix configured to relay through an authenticated provider is usually easier to operate than exposing an Internet-facing SMTP service. A direct client is convenient when you do not want a local queue, but you then need to handle transient failures in your script or accept that the remote relay is the only durable queue.
Method 1: send through local Postfix and mail
Install the packages
- Refresh the package index and install Postfix and Mailutils:
sudo apt update sudo apt install postfix mailutils - When the package configuration screen appears, choose the mail-system type that matches the server’s role. For a machine that only sends notifications, a relay or local-only design is safer than accepting mail from the public Internet. Use a fully Internet-facing configuration only when you intentionally operate an SMTP server and have completed its DNS, abuse-prevention and security setup.
- After changing Postfix settings, restart the service:
sudo systemctl restart postfix.service
Submit a plain-text test message
Pipe the body on standard input and set the subject with -s:
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printf '%sn' 'Body text from the server' | mail -s 'Ubuntu mail test' [email protected]You can supply multiple recipients as separate arguments. For a script, generate the body with a here-document or another command, but keep the same standard-input pattern so that shell quoting does not alter the message unexpectedly.
Check whether Postfix accepted it
Immediately inspect the service and queue:
systemctl status postfix
postqueue -pAn empty queue can mean that Postfix delivered the message onward quickly; it is not proof that the recipient saw it. A queued item with a deferred status indicates that Postfix has not completed delivery and will normally retry. Record the queue ID when you need to correlate a command with log entries.
Use the sendmail-compatible interface in scripts
Postfix also provides a sendmail-compatible command that reads a complete message from standard input. The -f option controls the envelope sender:
printf 'From: [email protected]: [email protected]: Ubuntu testnnBody textn' | sendmail -f [email protected] [email protected]The sendmail manpage warns that -- should terminate options before untrusted, user-supplied addresses. Use that boundary when your script accepts recipients from outside input. Diagnostics can appear on standard error and in syslog or the Postfix logging service, so capture both when troubleshooting.
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SendEmail is a command-line SMTP client. It connects to the host and port you specify instead of handing the message to a local Postfix queue. On Ubuntu Noble, the SendEmail manpage identifies version 1.56-5.2.
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Install and send a basic authenticated message
sudo apt update
sudo apt install sendemail
sendEmail -f [email protected] -t [email protected]
-u 'Ubuntu test' -m 'Body text' -s smtp.example.com:587
-xu 'smtp-user' -xp 'smtp-password'The documented options include -a for attachments, -cc and -bcc for additional recipients, -o for extended settings and -v for verbose protocol output. Port 587 commonly expects authenticated submission with STARTTLS, but the hostname, port, authentication method and sender-domain rules belong to your chosen relay. Add the TLS setting required by that relay through the installed client’s documented -o options; do not assume that using port 587 alone encrypts the session.
Keep the password out of history and process listings
The example uses -xp only to show the option. A real password on a command line can be recorded in shell history and may be visible to other users through process inspection while the command runs. Prefer a protected secret store or a file readable only by the account that sends mail. If a password was exposed during testing, rotate it and remove the old history entry where appropriate.
Method 3: use the Mailutils SMTP client
GNU Mailutils documents SMTP mailbox URLs such as smtp://user:pass@host:port, with optional authentication mechanisms. A URL is convenient for automation, but embedding a password in a URL has the same disclosure risks as putting it in a command argument.
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The Mailutils interactive client lets you observe each protocol stage:
mailutils smtpAt the prompt, the documented commands include:
connect smtp.example.com:587to open a connection.-tlsor the client’s TLS command to request encrypted transport where supported.ehlo mail.example.comto display the server’s advertised extensions.starttlsto upgrade a submission connection when the server offers it.authto authenticate with a supported mechanism.from [email protected]andto [email protected]to set the envelope addresses.sendto submit the message.
Mailutils’ current manual edition is 3.21, updated 12 October 2025. Exact prompts and authentication mechanisms can vary with the installed build and the relay’s advertised capabilities, so use the client’s built-in help if a command is rejected.
Configure Postfix to relay through authenticated SMTP
Relaying through a managed SMTP service lets local scripts use the familiar mail command while the provider handles Internet delivery. The provider’s current documentation is authoritative for its endpoint, port, authentication method, sender requirements and certificate policy.
Set the relay endpoint
Set relayhost to the provider’s SMTP endpoint. A bracketed host prevents Postfix from looking up MX records for that name:
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sudo postconf -e 'relayhost = [smtp.example.com]:587'For a different provider, substitute its documented host and port. Do not copy a port or hostname from an unrelated service; these values change and some providers require a region-specific endpoint.
Enable SASL authentication and TLS
SMTP-AUTH identifies the client through SASL, while TLS encrypts the session and protects credentials in transit. A typical Postfix relay configuration enables authentication and requires encrypted delivery:
sudo postconf -e 'smtp_sasl_auth_enable = yes'
sudo postconf -e 'smtp_tls_security_level = encrypt'
sudo postconf -e 'smtp_tls_CApath = /etc/ssl/certs'Authentication-map and certificate-verification settings must match the provider’s instructions and the authentication mechanism it advertises. A provider may require a submission port, an application-specific credential, a particular envelope sender or a verified domain.
Store credentials with restrictive permissions
One common Postfix arrangement is a password map readable only by Postfix:
sudoedit /etc/postfix/sasl_passwdEnter the provider’s documented map entry in the form smtp.example.com:587 smtp-user smtp-password, save the file, build the database and restrict both files:
sudo postmap /etc/postfix/sasl_passwd
sudo chown root:root /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
sudo chmod 600 /etc/postfix/sasl_passwd /etc/postfix/sasl_passwd.db
sudo systemctl restart postfix.serviceThe account that runs Postfix must be able to read the resulting map, but unrelated users should not. If your provider documents a different credential backend, use that instead. Never commit the password map to source control or place it in a world-readable backup.
Send through the relay
Once the relay is configured, the same local command works:
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printf '%sn' 'Relay test from Ubuntu' | mail -s 'Authenticated SMTP test' [email protected]Watch the log while submitting so you can distinguish local acceptance, authentication, TLS negotiation and remote rejection:
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sudo journalctl -u postfix -fTest SMTP and TLS without sending a full message
Ubuntu’s Postfix guide demonstrates checking SMTP capabilities with Telnet:
telnet mail.example.com 25
ehlo mail.example.comTelnet is plaintext and should be used only where that is appropriate; never type real credentials into an unencrypted session. For a STARTTLS submission endpoint, use a TLS-aware diagnostic such as:
openssl s_client -starttls smtp -connect smtp.example.com:587 -crlfAfter the connection is established, issue EHLO mail.example.com. A correctly configured server may advertise capabilities such as 250-STARTTLS and 250-AUTH. If STARTTLS is absent, stop and verify the port and provider documentation rather than sending credentials. Certificate errors, an unexpected hostname or a failed TLS upgrade indicate a configuration or endpoint problem that should be fixed before testing authentication.
Troubleshoot a message that did not arrive
1. Confirm the local service and queue
- Check the service:
systemctl status postfix. - List queued mail:
postqueue -p. - Look for a deferred item, a bounce, or a queue ID that you can trace.
If Postfix is stopped, start it and then inspect why it failed before resubmitting repeatedly. Repeated retries can create duplicate messages once the original eventually leaves the queue.
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Use the systemd journal for the service:
sudo journalctl -u postfix --since '15 minutes ago'On systems where it is enabled, the traditional system mail log contains the same delivery conversation:
sudo tail -n 100 /var/log/mail.logSearch for the queue ID, recipient address, SMTP status code, authentication failure, TLS error or remote rejection text. A local “queued” line means Postfix accepted the message; a later “status=sent” line means the next SMTP hop accepted it, not necessarily that the final mailbox displayed it.
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3. Check network and name resolution
- Verify that the server resolves the relay hostname and the recipient domain.
- Check outbound firewall rules and whether the hosting network blocks TCP port 25 or requires submission on 587 or 465.
- Confirm the server’s hostname and reverse DNS. Poor or missing reverse DNS can cause remote deferral or rejection.
- Check the sender domain’s SPF, DKIM and DMARC policy. A relay may require domain alignment or refuse an unverified sender.
4. Check authentication and provider policy
- Confirm the username, credential type and endpoint are correct for the selected region or account.
- Ensure the password map is readable by Postfix but not by other users.
- Read the provider’s exact SMTP rejection text; it may identify a blocked sender, unverified domain, rate limit or prohibited recipient.
- Repeat the EHLO and STARTTLS capability test on the same host and port that Postfix uses.
5. Rotate anything exposed during testing
If a password appeared in shell history, a process listing, a transcript or a world-readable file, treat it as compromised. Revoke or rotate it at the provider, replace the protected map, restart Postfix and remove unnecessary copies. Do not solve an authentication problem by disabling TLS or certificate verification.
Reliable patterns for cron and system scripts
Use a controlled envelope sender
Set a sender address from a domain you control and that your relay permits. The visible From: header and the SMTP envelope sender are related but not identical; a relay can reject one even when the other looks valid. The sendmail-compatible -f option controls the envelope sender, while a complete message can include its own headers.
Capture errors and preserve queue IDs
Scripts should log the command’s exit status and, for Postfix submissions, retain the queue ID from the journal when possible. A zero exit code means the submission step succeeded. It does not guarantee remote acceptance, inbox placement or human reading. For important alerts, monitor the queue and provider bounces instead of relying on the initial command result.
Attach files only when needed
SendEmail documents -a for attachments. Keep attachment size within the relay’s documented limit, use a content type appropriate to the file and avoid sending secrets in clear-text attachments. For routine monitoring, a short text body is more portable and less likely to trigger size or filtering limits.
Security and deliverability checklist
- Use authenticated SMTP submission and TLS for external relays.
- Keep credential files readable only by the account or service that needs them.
- Do not put real passwords in shell history, shared scripts, process arguments, repositories or backups.
- Verify certificates and hostnames; do not disable verification to bypass an error.
- Use a sender domain with correct SPF, DKIM and DMARC alignment where the relay supports it.
- Maintain accurate hostname and reverse-DNS records for servers that connect directly to recipient MX hosts.
- Limit outbound mail to the recipients and rate your application actually requires.
- Inspect Postfix logs and the queue after configuration changes.
- Rotate credentials immediately after accidental disclosure.
Which method should you use?
Choose local Postfix plus mail when several scripts, cron jobs or system services need a dependable local queue and one central relay policy. Choose SendEmail when a small script must connect directly to a known SMTP endpoint and can handle its own failures. Choose Mailutils’ SMTP client when SMTP URLs or interactive capability testing are more useful than a local queue.
In every case, separate three outcomes: the shell client ran, the local MTA or remote SMTP server accepted the message, and the recipient’s mailbox delivered it. Only the last outcome answers whether the reader actually received the mail, and it must be verified through queue records, SMTP responses, bounces and recipient-side evidence rather than a successful command alone.
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