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On Ubuntu, the most reliable command-line approach is to submit mail to Postfix, Ubuntu’s default supported mail-transfer agent, with the mail command. For a server that must use an external provider, either configure Postfix as an authenticated SMTP relay or connect directly with SendEmail or Mailutils. In every case, use authenticated SMTP and TLS for Internet delivery, protect credentials, and check the queue and logs: a command that exits successfully proves submission, not that the message reached the recipient’s inbox.
This guide shows the local Postfix path first, then authenticated relaying and direct SMTP clients, with commands, security precautions, testing methods, and recovery steps.
How command-line mail works on Ubuntu
A shell command is normally a Mail User Agent (MUA). It hands the message to a Mail Transfer Agent (MTA), which queues it, applies transport rules, and delivers it locally or to another SMTP server. A Mail Delivery Agent (MDA) may place locally delivered mail into a mailbox, and a POP3 or IMAP server lets users retrieve it. Ubuntu’s Server documentation describes this chain and states: “On Ubuntu, Postfix is the default supported MTA.”
That separation explains why two commands can look similar but behave differently. mail usually talks to a local MTA and benefits from its queue, retry policy, and centralized logs. SendEmail can open an SMTP connection itself, so the relay hostname, port, credentials, and TLS behavior are specified at invocation time. Mailutils can do either scripted SMTP submission or an interactive SMTP session.
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Choose the right sending method
| Method | Best fit | Queue and retries | Authentication and TLS | Operational visibility |
|---|---|---|---|---|
Postfix with mail or Mailutils |
Cron jobs, monitoring, and system scripts on a host with a controlled MTA | Postfix queues messages and retries according to its transport policy | Configured centrally, including authenticated relay and TLS policy | Postfix queue commands and service logs |
| SendEmail | A small script or one-off command that must connect directly to a specified SMTP endpoint | The client submits directly; any later retry behavior belongs to the SMTP service | Command options specify the server, account, password, and other SMTP settings; use a protected secret mechanism and TLS supported by the relay | Verbose client output plus the relay’s logs |
| Mailutils SMTP | SMTP URL parameters or interactive protocol diagnostics | Depends on whether it submits through Postfix or connects directly | Supports SMTP URLs, authentication mechanisms, and interactive TLS commands | Interactive protocol responses are useful for troubleshooting |
For recurring server notifications, local Postfix is usually the easiest to operate because applications do not need to store SMTP credentials. For a quick test against a managed relay, SendEmail is more explicit. Use Mailutils when you need to inspect an SMTP conversation or express connection details as an SMTP mailbox URL.
Send a message through local Postfix
Install Postfix and Mailutils
Update the package index and install the MTA and command-line mail interface:
sudo apt update
sudo apt install postfix mailutilsThe package configuration screen asks for a mail-system type. Select a design that matches the server’s role. A send-only host generally needs a relay or local-only arrangement rather than accepting mail from the Internet. Do not choose an Internet-facing configuration simply because the server needs to send alerts; accepting inbound mail creates additional exposure and administration.
Send the first test message
Once Postfix is installed and configured, pipe the body to mail and provide a subject with -s:
printf '%sn' 'Body text from the server' | mail -s 'Ubuntu mail test' recipient@example.comThe command submits the message to the local mail interface. Postfix then places it in its queue and attempts delivery using its current transport configuration. Replace the recipient with an address you control while testing. Avoid putting confidential data in a test message.
Check the service and queue
Confirm that Postfix is running and inspect queued messages immediately after a test:
systemctl status postfix
postqueue -pAn empty queue can mean the message was handed off successfully, but it is not proof of inbox placement. A queued item normally includes an ID and a reason for the delay, such as a connection, authentication, DNS, or recipient-policy failure. Keep that ID when examining logs or opening a support case with the relay.
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After changing Postfix configuration, restart the service:
sudo systemctl restart postfix.serviceRead the service journal and, where enabled by the system’s logging configuration, the mail log:
journalctl -u postfix
sudo tail -f /var/log/mail.logThe journal command works on systems using systemd. The traditional mail log may not exist if that logging facility is not enabled, so treat it as an additional location rather than the only source of evidence.
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When relaying is preferable
Direct delivery from a cloud or residential IP is often rejected because of reverse-DNS, reputation, sender-domain, or recipient-policy checks. A managed SMTP relay centralizes those requirements and usually provides authenticated submission on a dedicated port. Postfix remains the local queue and retry engine, while the relay performs onward delivery.
Set the relay endpoint and SMTP authentication
Configure Postfix’s relayhost to the SMTP endpoint supplied by your provider. Then configure SASL credentials, TLS policy, and certificate verification according to that provider’s current documentation. SMTP-AUTH identifies the client through SASL and TLS; after successful authentication, the server can authorize relaying.
Provider-specific hostnames, ports, authentication mechanisms, and sender-domain requirements change. Verify those values in the provider’s current documentation instead of copying an example intended for another service. A submission port commonly differs from an unauthenticated server-to-server port, and a provider may require a particular TLS mode or sender address.
Protect the credential store
- Keep the SASL password file readable only by the account or service that needs it. Check ownership and permissions after creating or changing it.
- Do not place a real password in a shared shell command, a ticket, a public script, or shell history.
- Use a dedicated relay identity with the least privilege available, and rotate it if it has appeared in a process list, terminal recording, log, or source repository.
- Use certificate verification and an encrypted SMTP session. Do not downgrade to plaintext merely because a diagnostic command is easier.
Restart and verify the relay
Restart Postfix after changing the relay configuration, send a test through mail, and inspect both the queue and logs. The log should show a successful authenticated handoff to the relay or a precise failure returned by it. If the message remains queued, fix the reported cause before sending repeated tests; otherwise you can create a backlog and trigger provider rate limits.
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Use the Postfix sendmail-compatible interface
Postfix provides a sendmail-compatible command for programs that write a complete message to standard input. The -f option controls the envelope sender:
printf 'From: sender@example.comnTo: recipient@example.comnSubject: Ubuntu sendmail testnnBody textn' | sendmail -f sender@example.com recipient@example.comThe compatibility interface accepts message input on standard input and reports diagnostics through standard error and the system’s mail logging path. When an address comes from an untrusted variable, terminate options with -- before the address so a value beginning with a hyphen cannot be interpreted as another option:
printf '%sn' 'Body text' | sendmail -f sender@example.com -- "$recipient"Use a properly formatted message, including a blank line between headers and the body. The envelope sender set by -f is separate from the visible From: header and may be subject to the relay’s sender-domain policy.
Connect directly with SendEmail
Install and check the client
On Ubuntu, the SendEmail package provides a lightweight command-line SMTP client. Install it with APT if it is available in the enabled repositories, then check the installed manual page for the exact version and options. The Ubuntu Noble manual identifies SendEmail version 1.56-5.2.
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sudo apt update
sudo apt install sendemail
man sendEmailSubmit a message to an authenticated relay
The documented form supplies the sender, recipient, subject, body, SMTP endpoint, username, and password:
sendEmail -f sender@example.com -t recipient@example.com
-u 'Ubuntu test' -m 'Body text' -s smtp.example.com:587
-xu 'smtp-user' -xp 'smtp-password'The example demonstrates the option names; replace every value with the relay’s approved settings. Do not use a real password in a shared command history or in a script readable by other users. Prefer a protected secret mechanism supported by your operating environment and the relay. If the client’s default encryption behavior does not match the provider’s requirements, set the documented TLS-related extended options with -o and verify the result in verbose mode.
Useful SendEmail options
-aattaches a file. Confirm that the file is permitted to leave the server and that its size complies with relay limits.-ccand-bccadd copied recipients. Remember that blind-copy addresses are still visible to the SMTP service and may be logged.-osupplies extended settings, including options needed for a relay’s TLS or authentication behavior.-venables verbose diagnostics. Use it during testing, but review the output for exposed credentials before saving or sharing it.
SendEmail’s success status means the client completed its submission attempt. It does not establish that the recipient accepted the message permanently or that it appeared in the inbox.
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Use Mailutils for SMTP URLs and interactive diagnostics
SMTP mailbox URLs
GNU Mailutils documents SMTP mailbox URLs in the form smtp://user:pass@host:port, with optional authentication mechanisms. Treat a URL containing a password as a secret: it can be exposed through shell history, process inspection, logs, or copied terminal output. Use an alternative protected credential mechanism where your deployment supports one.
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The current GNU Mailutils manual is edition 3.21 and was updated 12 October 2025. Confirm the installed package’s manual page because command syntax and available authentication mechanisms depend on the version installed on the server.
Interactive SMTP session
The mailutils smtp client can connect to a server and expose protocol steps directly:
mailutils smtp
connect smtp.example.com 587
-tls
ehlo mail.example.com
auth
from sender@example.com
to recipient@example.com
sendUse the exact interactive syntax shown by mailutils smtp --help for the installed release. The important diagnostic sequence is to connect, negotiate TLS when required, issue EHLO, authenticate, specify the envelope sender and recipient, and submit the message. Never type a production password into a session that is being recorded or shared.
Test SMTP capabilities safely
Ubuntu’s Postfix guide demonstrates capability testing with telnet mail.example.com 25 followed by ehlo mail.example.com. This can show server capabilities such as 250-STARTTLS and 250-AUTH when enabled, but Telnet itself is plaintext. Use it only where exposing the session is acceptable and never send credentials through an unencrypted connection.
For a submission port or a server that requires encryption, use a TLS-aware client that performs certificate verification and displays the negotiated capabilities. Check that the certificate name matches the hostname, that the server advertises the authentication method you intend to use, and that the connection fails rather than silently falling back to plaintext when TLS is mandatory.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Understand what a successful command proves
There are several separate milestones:
- Local submission: the command passed a syntactically acceptable message to Postfix or another local process.
- Queue acceptance: the MTA assigned an ID and retained the message for delivery.
- SMTP handoff: a destination or relay server accepted the message during an SMTP transaction.
- Final delivery: the recipient’s system stored the message in a mailbox rather than rejecting, quarantining, or filtering it.
A zero exit status generally covers only the first milestone, and sometimes the second. Confirm later stages through Postfix logs, relay responses, bounce messages, and the recipient’s mailbox or provider console. There is no universal Ubuntu command-line delivery-success percentage: results vary with DNS, authentication, IP reputation, sender policy, relay rules, and recipient policy.
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Troubleshoot a message that did not arrive
1. Check the service and queue
systemctl status postfix
postqueue -pIf Postfix is stopped, restart it and inspect the journal for the startup error. If a message is queued, record its queue ID and the reason shown beside it. A growing queue indicates a systemic problem rather than a single recipient typo.
2. Read the relevant logs
journalctl -u postfix
sudo tail -f /var/log/mail.logRun the log view while submitting one test message so you can correlate the timestamp, queue ID, remote host, SMTP response, and retry decision. Look for authentication failures, TLS negotiation errors, DNS lookup failures, connection timeouts, and recipient rejection codes.
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- Confirm that the server can resolve the relay hostname and that outbound DNS is permitted.
- Check the server hostname and the reverse-DNS record for the sending IP where the relay or recipient policy requires them.
- Verify that the sender domain’s SPF, DKIM, and DMARC policy is compatible with the relay and the visible envelope sender.
- Make sure the TLS certificate is valid for the hostname you configured, not merely for an IP address.
4. Validate authentication and sender policy
Recheck the relay username, password, port, authentication mechanism, and required encryption mode. Some relays permit authentication only after TLS. Others allow a login but reject a sender address that is not authorized for the account or verified domain. Use the exact rejection text from the SMTP response rather than guessing.
5. Distinguish rejection, deferral, and filtering
A permanent SMTP rejection should be corrected before retrying. A temporary deferral may clear on a later Postfix retry, but repeated deferrals can indicate a blocked port, DNS problem, rate limit, or remote outage. If the relay accepted the message and Postfix removed it from the queue, investigate the relay’s delivery event and the recipient’s spam or quarantine folders; local queue status alone cannot show what happened afterward.
6. Rotate exposed credentials
If a password appeared in a command line, process listing, shell history, transcript, or log, rotate it immediately, update the protected credential store, and restart or reload the service as required. Remove copies from scripts and shared files, then send a new test with a non-sensitive body.
Operational practices for scripts and cron jobs
- Use a stable local Postfix submission path for recurring jobs so scripts do not each carry SMTP credentials.
- Set an explicit envelope sender that your relay permits, and use a consistent subject that identifies the host and job.
- Keep message bodies free of passwords, access tokens, and unnecessary personal data.
- Capture command errors and alert when the Postfix queue exceeds a reasonable operational threshold.
- Test after package upgrades or relay-policy changes; a previously working command can fail when TLS, authentication, or sender requirements change.
- Restrict who can read scripts, environment files, queue data, and log exports, because those locations can contain addresses and message content.
Practical decision guide
Choose local Postfix when
The server runs cron or monitoring jobs, you want queueing and retries, and an administrator can maintain one centrally protected relay configuration. Applications submit with mail or the Postfix-compatible interface without handling SMTP passwords.
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You need a short, direct SMTP invocation for a controlled test or a small utility and the relay’s endpoint and authentication requirements are known. Use verbose mode only during diagnosis and protect the credential source.
Choose Mailutils SMTP when
You need SMTP URL parameters, selectable authentication mechanisms, or an interactive session that exposes each protocol step. It is particularly useful for isolating whether failure occurs during connection, TLS negotiation, authentication, envelope commands, or message transfer.
Whichever path you select, separate submission from delivery in your diagnosis: inspect the queue, read the logs, verify TLS and authentication, and use the remote server’s response to determine the next fix.
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