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What happens when you send mail from a terminal
A shell command is not usually the component that delivers mail across the Internet. It hands a message to a mail user agent (MUA) or SMTP client. A mail-transfer agent (MTA), normally Postfix on Ubuntu, accepts the message, places it in a queue, looks up the destination, and attempts delivery. A mail-delivery agent (MDA) can then place the message in a mailbox that the recipient reads through POP3 or IMAP.
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Ubuntu’s server documentation describes this MUA–MTA–MDA–retrieval chain and states: “On Ubuntu, Postfix is the default supported MTA.” That distinction explains why a local command can return successfully even when the recipient later sees nothing: the command may have only handed the message to the local queue, or to an SMTP server that accepted it for further processing.
Choose the right command-line route
| Approach | Best fit | Queueing and retries | Authentication and TLS | Diagnostics |
|---|---|---|---|---|
Postfix with mail/Mailutils |
Cron jobs, monitoring, and system scripts on a host with a controlled local MTA | Postfix queues mail and retries according to its transport rules | Configured centrally in Postfix; suitable for an authenticated relay | Postfix queue commands and service logs |
| SendEmail | A small, explicit SMTP submission that names the relay, sender, recipient, and credentials | The SMTP server receives the message; local queueing is not provided unless you add it separately | Set the relay’s TLS mode and authentication options; avoid exposing the password | Use -v for a protocol-level conversation |
| Mailutils SMTP client | SMTP URL parameters or interactive protocol troubleshooting | Depends on the SMTP server unless used through a local MTA | Supports SMTP URLs, authentication mechanisms, and interactive STARTTLS | Interactive commands such as ehlo, starttls, and auth |
For recurring server-generated mail, local Postfix is generally the most resilient design because the queue survives a temporary network or relay outage. For a one-time test against a known relay, a direct SMTP client is quicker. Do not choose a method solely because its command is shorter; decide whether you need a durable queue, centralized logs, and one protected credential store.
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Prerequisites and safety checks
- Use a supported Ubuntu server account with
sudoaccess for package installation and Postfix configuration. - Know whether the host should send only outbound mail, relay through a provider, or accept mail from other systems. A send-only host should not be configured as an open Internet-facing MTA.
- Obtain the relay’s current hostname, port, TLS requirement, authentication method, permitted sender domains, and certificate expectations from that provider. These values vary and are not universal.
- Choose a sender address in a domain you control. The relay or recipient can reject arbitrary From addresses even when SMTP authentication succeeds.
- Keep credentials out of shared shell history, scripts readable by other users, process listings, backups, and world-readable configuration files.
Before changing a production configuration, record the current state with postconf -n and save a copy of any file you edit. Test from a non-critical recipient mailbox and remove or rotate credentials if they are exposed during a trial.
Send mail through local Postfix and mail
Install Postfix and Mailutils
Refresh package metadata and install the MTA plus the command-line mail interface:
sudo apt update
sudo apt install postfix mailutilsPackage configuration asks for a mail-system type. Select the option that matches the server’s role. A machine that only submits outbound notifications normally uses a local-only or relay design rather than accepting mail directly from the Internet. If you are unsure, stop and determine whether another service already owns SMTP ports or whether this host is expected to receive mail; an incorrect choice can expose an unintended relay.
Submit a plain-text test
Once Postfix is configured, pipe the body on standard input and set the subject with -s:
printf '%sn' 'Body text from the server' | mail -s 'Ubuntu mail test' [email protected]Replace the recipient with an address you can inspect. The command invokes Mailutils, which submits to the local MTA. It does not wait for the final recipient mailbox. A zero exit status means the local submission step completed; it is not a delivery receipt.
Check the queue and service state
Immediately inspect Postfix:
systemctl status postfix
postqueue -pAn empty queue can mean the message was delivered onward or that it was rejected before queueing. A queued item normally shows an identifier and a reason for deferred delivery. After correcting a configuration problem, Postfix will retry deferred messages; you can request a delivery attempt with sudo postqueue -f once the underlying issue is fixed.
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Read the relevant logs
Use the service journal:
journalctl -u postfixOn installations that write a traditional mail log, inspect the system mail log as well. Search by the queue ID shown by postqueue -p. Entries such as status=sent, status=deferred, or status=bounced tell you whether the next hop accepted the message, temporarily delayed it, or rejected it. A remote server’s acceptance still does not guarantee placement in the recipient’s inbox.
Use the Postfix sendmail compatibility interface
Postfix supplies a sendmail-compatible command that reads a complete message from standard input. It is useful when a script needs explicit headers or when existing software expects a sendmail binary:
printf 'From: [email protected]: [email protected]: Ubuntu testnnBody textn' | sendmail -f [email protected] [email protected]The -f option controls the envelope sender, which is distinct from the visible From: header. Keep those identities consistent unless you have a deliberate bounce-handling design. The sendmail manpage warns that -- should terminate options before untrusted, user-supplied addresses; this prevents an address beginning with a hyphen from being interpreted as another option. For example, a program that accepts a recipient from a user should place -- before that value.
Diagnostics can be written to standard error and to syslog or Postfix’s logging service. Do not treat the compatibility command as a direct Internet SMTP client: it still submits to the local Postfix instance, so relay settings, queue policy, and logs remain the source of truth.
Configure Postfix to relay through authenticated SMTP
A managed relay is usually preferable to direct-to-recipient delivery from a small server. The relay handles the provider’s policy and often improves deliverability, but it requires the exact endpoint and security settings supplied by that provider.
Set the relay host and TLS policy
Postfix settings are normally stored in /etc/postfix/main.cf. Set relayhost to the provider’s SMTP endpoint, including its submission port when required. Configure SASL authentication, require encryption for the authenticated session, and enable certificate verification according to the provider’s current instructions. A typical configuration shape is:
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relayhost = [smtp.example.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
smtp_tls_CApath = /etc/ssl/certsThe hostname, port, TLS level, and certificate settings above are examples of the settings to discuss with the relay provider; do not copy them blindly. Some providers require a different port, a specific authentication mechanism, or a sender-domain policy. Brackets around the host in relayhost prevent an MX lookup and make Postfix connect to that exact endpoint.
Store the SMTP credential in a protected map
Create a root-owned password map whose key exactly matches the relay host and port:
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sudo sh -c 'umask 077; printf "%sn" "[smtp.example.com]:587 smtp-user:smtp-password" > /etc/postfix/sasl_passwd'
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.dbUse a dedicated relay credential where the provider supports one, and grant it only the permission needed to send. The plaintext map is used to generate the database; both files must be protected. Do not place a real password in a command recorded by a shared shell history or visible to another user. If a password has appeared in history, a process list, a ticket, or a log, rotate it at the provider before continuing.
Reload and test
After changing main.cf or the credential map, validate the effective settings and restart Postfix:
sudo postconf -n
sudo systemctl restart postfix.service
printf '%sn' 'Authenticated relay test' | mail -s 'Postfix relay test' [email protected]Watch journalctl -u postfix while submitting. Authentication failures usually indicate a wrong username, password, map key, or provider-required mechanism. TLS failures point to the endpoint, port, system clock, certificate trust, or an encryption policy mismatch. A relay can also accept authentication and then reject the envelope sender or recipient under its account policy.
Send directly with SendEmail
SendEmail is suited to a compact invocation that connects to a specified SMTP server rather than relying on the local Postfix queue. The Ubuntu Noble manpage identifies SendEmail version 1.56-5.2 and documents this form:
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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 options set the envelope sender (-f), recipient (-t), subject (-u), body (-m), SMTP endpoint (-s), username (-xu), and password (-xp). SendEmail also documents -a for attachments, -cc and -bcc for additional recipients, -o for extended settings, and -v for verbose protocol output.
The command as printed contains a password and is therefore appropriate only as a syntax illustration. In production, use the client’s supported protected-secret and TLS mechanism, or invoke it from a root-readable wrapper that does not expose the secret in arguments. Command-line arguments can be visible through process inspection while the program runs, and shell history can persist after it exits. Confirm the relay’s STARTTLS or implicit-TLS mode; merely selecting port 587 does not by itself prove that encryption is enabled.
Run a verbose test only when the output can be handled safely:
sendEmail -v -f [email protected] -t [email protected]
-u 'Verbose SMTP test' -m 'Inspect the session and remove sensitive output'
-s smtp.example.com:587 -xu 'smtp-user' -xp 'smtp-password'Review the transcript for the server greeting, the EHLO response, STARTTLS negotiation, authentication result, and final acceptance. Redact usernames or tokens before storing the transcript.
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GNU Mailutils documents SMTP mailbox URLs in the form smtp://user:pass@host:port, with optional authentication mechanisms. Its current manual edition is Mailutils 3.21, updated 12 October 2025. Embedding a password in a URL has the same exposure risk as putting it in a command argument, so use a protected method whenever the client and provider support one.
The interactive mailutils smtp client is useful when you need to observe each stage of an SMTP conversation. The documented command set includes:
connectto open a session to the chosen server.-tlsto request the client’s TLS mode where supported.ehloto display capabilities advertised by the server.starttlsto upgrade a plaintext connection to TLS.authto authenticate using a supported mechanism.fromandtoto set the envelope addresses.sendto submit the message.
Use this mode to determine whether the relay advertises 250-STARTTLS, 250-AUTH, or another capability expected by your configuration. Do not authenticate over an unencrypted connection unless the provider explicitly documents that design and the network is trusted.
Test SMTP capabilities safely
Ubuntu’s Postfix guide demonstrates connecting to port 25 with Telnet and issuing ehlo mail.example.com. That is useful for inspecting a server that permits a plaintext capability check, but Telnet does not provide encryption. Prefer a TLS-aware diagnostic client when credentials or message content could be transmitted, and follow the relay provider’s port-specific instructions.
The EHLO response tells you what the server supports. A server that does not advertise STARTTLS on the selected port cannot satisfy a policy that requires STARTTLS there; using a different submission port may be necessary. Likewise, an absent AUTH capability can mean that authentication is intentionally disabled until TLS starts, not that the account is invalid.
Diagnose messages that do not arrive
1. Confirm local submission
Run systemctl status postfix, submit a test, and check postqueue -p. If Postfix is stopped, fix the service before investigating DNS or recipient policy. If the queue ID appears, use it to follow the message through the journal.
2. Distinguish deferred, bounced, and accepted
- Deferred: Postfix will retry. Common causes include a temporary DNS failure, a refused connection, rate limiting, or a transient 4xx SMTP response.
- Bounced: The message was rejected permanently or retries were exhausted. Read the remote diagnostic text before changing settings.
- Sent or accepted by the relay: The next SMTP server took responsibility. Inbox placement can still be affected by recipient filtering, sender authentication, reputation, or mailbox rules.
3. Check DNS and host identity
Verify that the server resolves the relay hostname and that its system clock is correct for certificate validation. For direct delivery, the sending IP’s reverse DNS, forward-confirmed hostname, and network reputation matter. A relay may require the envelope sender’s domain to have SPF, DKIM, and DMARC alignment; those policies are domain-specific and must be checked with the domain administrator.
4. Read the exact SMTP rejection
Do not replace a remote error with a generic retry. A response identifying invalid credentials calls for a credential or account-policy fix. A response naming an unauthorized sender requires a permitted From domain or address. A response about recipient policy belongs to the receiving organization, not to the local mail command. Preserve the queue ID and complete diagnostic text when escalating to the relay provider.
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Once the endpoint, credentials, DNS, or policy is corrected, request a queue run with sudo postqueue -f. Repeatedly flushing a queue without correcting the cause can create needless load and may trigger provider rate limits.
Security practices that prevent common failures
- Use authenticated SMTP with TLS for external relays. Never assume that a network path is private because the server is in a data center.
- Keep SASL maps, wrapper scripts, and logs readable only by the account that needs them. Review permissions after package upgrades or configuration management runs.
- Use separate credentials for separate hosts or applications so one compromise does not expose every sender.
- Limit the envelope sender to addresses and domains authorized by the relay. Align visible
From:headers with the authenticated sending identity where policy requires it. - Remove test messages containing secrets, tokens, or personal data from scripts and log archives.
- Rotate any credential that appeared in shell history, process output, a debug transcript, or a support ticket.
Automate recurring notifications responsibly
Cron and monitoring jobs should normally call mail or the sendmail-compatible interface and let Postfix queue and retry. Include a stable subject that identifies the job, send from a controlled address, and capture the command’s exit status for local alerting. Do not create a new SMTP connection strategy in every script; centralizing relayhost, TLS, authentication, and logging in Postfix makes rotation and troubleshooting consistent.
For scripts that must use a direct relay, keep the message body in standard input or a protected file, pass recipients from a validated allowlist, and ensure the process account cannot read unrelated secrets. Test both success and failure paths: an SMTP server can be reachable while rejecting the sender, and a local command can succeed while a queued message later bounces.
Quick Recap
A practical decision checklist
- Choose local Postfix plus
mailwhen the server runs cron, monitoring, or system jobs and you want queueing, retries, and one central log. - Choose Postfix with an authenticated relay when an external provider supplies SMTP credentials and you want the operating system to manage TLS and delivery attempts.
- Choose SendEmail for a small, explicit SMTP submission when you can protect the credential and do not need a local retry queue.
- Choose Mailutils SMTP when URL parameters or an interactive session will help inspect EHLO capabilities, STARTTLS, and authentication.
- Regardless of the route, verify the queue or SMTP response, inspect logs, and confirm receipt independently. No universal delivery-success percentage applies to Ubuntu command-line mail; outcomes depend on DNS, authentication, relay policy, IP reputation, and recipient policy.
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