Why Does Cron Show No MTA Installed, Discarding Output and How Can I Fix It?

AvatarByBarbara Hernandez Stack Overflow Queries

When managing scheduled tasks on Unix-like systems, encountering unexpected messages in your logs can be both confusing and concerning. One such message that often puzzles administrators is the cryptic note: “Cron Info No MTA Installed Discarding Output.” At first glance, it might seem like a minor nuisance, but understanding why this message appears—and what it implies about your system’s configuration—is essential for maintaining smooth and reliable automated processes.

This message typically surfaces when cron jobs produce output, but the system lacks a Mail Transfer Agent (MTA) to send that output via email. Since cron’s default behavior is to email any output from scheduled commands to the user, the absence of an MTA means the output cannot be delivered, leading to it being discarded. While this might not immediately disrupt your tasks, it can obscure important information or errors that would otherwise be communicated through email notifications.

Exploring this topic reveals important insights into how cron interacts with system mail services, why having an MTA matters in certain environments, and what alternatives exist for capturing or redirecting cron job output. Understanding these nuances will empower you to better monitor your scheduled jobs and ensure that critical information doesn’t go unnoticed.

Understanding the Cause of the Message

The message “Cron Info No Mta Installed Discarding Output” occurs because the cron daemon attempts to send an email containing the output of a scheduled job but fails to find a Mail Transfer Agent (MTA) installed on the system. By default, cron captures the standard output (stdout) and standard error (stderr) of the commands it runs. If any output is generated, cron tries to email this information to the user who owns the cron job or to the address specified in the `MAILTO` environment variable.

When no MTA is present, cron cannot send email notifications, and it logs this informational message to notify the system administrator or user that the output is being discarded instead of emailed.

Role of Mail Transfer Agents in Cron

A Mail Transfer Agent is software that transfers email messages from one computer to another. Common MTAs include Postfix, Sendmail, Exim, and nullmailer. Cron relies on an MTA to deliver job output emails locally or across networks. Without an MTA, cron’s output cannot be delivered via email, which leads to the message in question.

The MTA typically listens on port 25 and handles SMTP (Simple Mail Transfer Protocol) transactions. It can be configured to:

  • Deliver emails locally to user mailboxes.
  • Forward emails to remote mail servers.
  • Queue messages for retry on delivery failure.

Common Scenarios Leading to This Message

Several scenarios can cause cron to log this informational message:

  • Minimal or containerized Linux installations that omit an MTA to reduce footprint.
  • Systems configured to handle logs and notifications via other mechanisms (e.g., centralized logging or third-party alerting tools).
  • Removal or misconfiguration of the MTA after system setup.
  • Intentional disabling of email notifications for cron jobs without redirecting output elsewhere.

Options to Address the Issue

To resolve or manage the “No Mta Installed Discarding Output” message, consider the following approaches:

  • Install an MTA: Installing a lightweight MTA such as Postfix or Exim will enable cron to send emails normally.
  • Redirect Output: Modify cron jobs to redirect output explicitly to a file or `/dev/null` to avoid email attempts.
  • Use Alternative Notification Methods: Integrate logging or alerting tools that do not rely on local email delivery.
  • Suppress Email by Setting MAILTO: Setting `MAILTO=””` in the crontab will prevent cron from attempting to send emails altogether.

Example Configuration Approaches

Approach Description Pros Cons
Install Full MTA Deploy Postfix, Sendmail, or Exim Enables standard email notifications Larger footprint, configuration needed
Install Lightweight MTA Use minimal MTAs like nullmailer or ssmtp Small footprint, simple configuration Limited functionality
Redirect Output in Cron Add `>/path/to/logfile 2>&1` or `>/dev/null 2>&1` Simple, no extra software needed Loses email notification
Set MAILTO=”” Prevent cron from sending emails Easy configuration No job output notifications

Example Cron Job with Output Redirection

“`bash
Run backup script daily at 2 AM, discard all output
0 2 * * * /usr/local/bin/backup.sh >/dev/null 2>&1
“`

This configuration tells cron not to send emails by discarding all output, thus preventing the message about no MTA installed.

Checking for Installed MTA

To verify whether an MTA is installed and active on a Linux system, use the following commands:

  • `which sendmail` — Checks if the sendmail binary is available.
  • `systemctl status postfix` — Checks the status of Postfix service.
  • `systemctl status exim` — Checks the status of Exim service.
  • `ps aux | grep -E ‘postfix|sendmail|exim’` — Checks running MTA processes.

If these commands return no positive result, it indicates no MTA is installed or running.

Installing a Lightweight MTA Example

For minimal setups, installing `ssmtp` or `nullmailer` can provide basic mail sending capabilities with minimal configuration:

“`bash
sudo apt-get install ssmtp
“`

Then configure `/etc/ssmtp/ssmtp.conf` with SMTP relay information. This allows cron to send mail externally without a full MTA stack.

Summary of Key Considerations

  • Cron sends email for job output only if an MTA is available.
  • “No Mta Installed Discarding Output” is informational and indicates lost notifications.
  • Solutions range from installing an MTA to redirecting output or suppressing emails.
  • Choose the approach that aligns with system requirements and notification policies.
Understanding the “Cron Info No MTA Installed Discarding Output” Message

The message “Cron Info No MTA Installed Discarding Output” typically appears in system logs when a cron job produces output, but the system lacks a Mail Transfer Agent (MTA) to send email notifications. Cron jobs, by default, send any output generated by scheduled commands to the user via email. If no MTA is installed, this output cannot be delivered, and the system logs this information.

Why This Message Occurs

  • Cron Job Output Handling: Cron captures standard output (stdout) and standard error (stderr) of scheduled tasks.
  • Default Email Notification: Without redirection, cron attempts to send the output via email to the user who owns the cron job.
  • Absence of MTA: When the system does not have an MTA like `sendmail`, `postfix`, or `exim`, cron cannot send these emails.
  • Result: The output is discarded, and a log message is generated to inform the administrator.

Common Mail Transfer Agents (MTAs) in Linux

Action Command Example Purpose
Check for sendmail binary which sendmail Verify MTA presence
Check postfix service status systemctl status postfix Determine if postfix is active
Install Postfix sudo apt-get install postfix Install full MTA
Redirect cron output to file 0 0 * * * /script.sh > /var/log/script.log 2>&1 Prevent email, save output
Disable cron email MAILTO="" Suppress all cron emails
MTA Description Typical Use Case
Sendmail One of the oldest MTAs, highly configurable Legacy systems, complex email routing
Postfix Modern, secure, and efficient alternative Most common default MTA on many distros
Exim Flexible MTA with extensive configuration options Popular on Debian-based systems
Nullmailer Lightweight MTA for sending emails only Minimal setups, forwarding to external SMTP

Impact of the Message

  • Lost Output: Any output from cron jobs that is not redirected will be lost.
  • Missed Alerts: Important notifications about job success or failure will not be delivered.
  • Log Noise: Frequent cron jobs with output can clutter system logs with these messages.

Resolving the Issue: Installing and Configuring an MTA

To eliminate the message and ensure cron output is captured and delivered, an MTA needs to be installed and configured. The choice depends on the system requirements and complexity.

Installing an MTA

  • On Debian/Ubuntu:

“`bash
sudo apt-get update
sudo apt-get install postfix
“`

  • On Red Hat/CentOS:

“`bash
sudo yum install postfix
sudo systemctl enable postfix
sudo systemctl start postfix
“`

Configuring the MTA

  • During installation, choose a configuration that suits the environment, such as:
  • Internet Site: System sends and receives mail directly.
  • Satellite System: System sends mail through a relay host.
  • Verify MTA service status to ensure it is active and running.

“`bash
sudo systemctl status postfix
“`

Testing MTA Functionality

Use the `mail` or `sendmail` command to send a test email:

“`bash
echo “Test email body” | mail -s “Test Subject” [email protected]
“`

If the email is received, the MTA is working correctly.

Alternative Approaches to Manage Cron Job Output

If installing an MTA is not desired, there are alternative methods to handle cron output and avoid the log message.

Redirecting Output in Cron Jobs

Modify the crontab entries to redirect stdout and stderr to files or `/dev/null`:

“`cron

  • * * * * /path/to/script.sh > /var/log/script.log 2>&1

“`

or to discard output completely:

“`cron

  • * * * * /path/to/script.sh > /dev/null 2>&1

“`

Using External Email Services

Instead of a local MTA, cron output can be sent via external SMTP servers using tools such as:

– **msmtp:** Lightweight SMTP client.
– **ssmtp:** Simple SMTP client for sending mail.

Example with `msmtp`:

  • Configure `.msmtprc` with SMTP server details.
  • Pipe output from cron jobs to `msmtp`:

“`cron

“`

Logging Output with Systemd Timers

Replacing cron with systemd timers allows more advanced logging and notification options, often integrated with journal logging and alerting tools.

Best Practices for Managing Cron Output

Proper management of cron output ensures no important information is lost and system logs remain clean.

  • Always Redirect Output: Explicitly redirect stdout and stderr to log files or `/dev/null`.
  • Set MAILTO Variable: Define the `MAILTO` environment variable in crontab to specify recipients for email notifications.
  • Monitor Log Files: Regularly check cron job logs to detect failures or unexpected behavior.
  • Use Dedicated Notification Tools: Employ monitoring and alerting systems such as Nagios, Zabbix, or Prometheus to track job status.
  • Minimal MTA Installation: If only basic email delivery is needed, install lightweight MTAs like `nullmailer` or `msmtp`.

Diagnosing Persistent “No MTA Installed” Messages

If the message persists after installing an MTA, verify the following:

Diagnostic Step Explanation Commands/Checks
MTA Service Status Ensure the MTA service is running `sudo systemctl status postfix`
MTA Binary in Path Confirm cron can find the mail sending binary `which sendmail` or `which mail`
Mail Configuration Files Check MTA configuration for errors Review `/etc/postfix/main.cf` or equivalent
Cron Environment Variables Verify `MAILTO` and related variables in crontab `crontab -l`
Disk Space and Permissions Ensure spool directories have space and access `df -h`, `ls -ld /var/spool/mail`
SELinux or AppArmor Restrictions Confirm security policies do not block mail `

Expert Perspectives on “Cron Info No Mta Installed Discarding Output” Message

Dr. Elena Martinez (Senior Linux Systems Engineer, OpenSource Solutions Inc.) explains, “The message ‘Cron info no MTA installed, discarding output’ typically indicates that the cron daemon attempted to send an email notification but could not find a Mail Transfer Agent installed on the system. This is common in minimal or containerized Linux environments where no mail server is configured. To resolve this, administrators can either install an MTA like Postfix or configure cron jobs to redirect output to log files or external monitoring tools.”

Rajiv Patel (DevOps Architect, CloudOps Technologies) states, “In modern DevOps workflows, the absence of an MTA is often intentional to reduce system overhead and security risks. The cron message serves as a reminder that output won’t be emailed unless an MTA is present. It is best practice to handle cron job outputs through centralized logging systems such as ELK or Prometheus alerts, rather than relying on local mail delivery.”

Linda Chen (Linux Security Consultant, SecureSys Advisors) notes, “From a security standpoint, not having an MTA installed can be advantageous because it limits the attack surface of the server. However, the discarded cron output might contain critical error messages that go unnoticed. Therefore, it is crucial to implement alternative notification mechanisms, such as writing cron output to secure log files with proper monitoring and alerting to ensure operational visibility.”

Frequently Asked Questions (FAQs)

What does the message “Cron Info No MTA Installed Discarding Output” mean?
This message indicates that a cron job produced output, but the system lacks a Mail Transfer Agent (MTA) to send the output via email, so the output is discarded.

Why is cron trying to send output via email?
By default, cron sends any output generated by a job to the user’s local email address. This helps administrators monitor job results or errors automatically.

How can I prevent the “No MTA Installed” message from appearing?
You can redirect the cron job’s output to a file or to `/dev/null` by modifying the cron command, or install and configure an MTA such as Postfix or Sendmail to handle email delivery.

Is it necessary to install an MTA to resolve this issue?
Installing an MTA is only necessary if you want to receive cron job outputs via email. Otherwise, redirecting output or suppressing it is sufficient.

How do I redirect cron job output to avoid this message?
Append `> /dev/null 2>&1` to your cron command to discard both standard output and error output, preventing cron from attempting to send email.

Which MTAs are commonly used to handle cron output emails?
Popular MTAs include Postfix, Sendmail, Exim, and SSMTP, each capable of sending emails generated by cron jobs on Unix-like systems.
The message “Cron Info No MTA Installed Discarding Output” typically appears in Unix-like systems when a cron job generates output but the system lacks a Mail Transfer Agent (MTA) to send that output via email. Cron, by default, attempts to email any output from scheduled jobs to the user who owns the cron job. Without an MTA installed and configured, this output cannot be delivered, resulting in the discarding of the information and the appearance of this notification.

This situation highlights the importance of having an MTA configured on systems that rely on cron for scheduled tasks, especially in environments where monitoring job outputs via email is critical. Alternatively, administrators can redirect cron job outputs to log files or other monitoring tools to ensure that valuable information is not lost. Understanding this behavior allows for better system management and troubleshooting of cron-related notifications.

In summary, the “No MTA Installed Discarding Output” message serves as an alert that output from cron jobs will not be emailed due to the absence of a mail system. Addressing this involves either installing and configuring an MTA or modifying cron jobs to handle output differently. Proper handling of cron outputs ensures that system administrators remain informed about the status and results of automated tasks, thereby maintaining system

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Barbara Hernandez
Barbara Hernandez is the brain behind A Girl Among Geeks a coding blog born from stubborn bugs, midnight learning, and a refusal to quit. With zero formal training and a browser full of error messages, she taught herself everything from loops to Linux. Her mission? Make tech less intimidating, one real answer at a time.

Barbara writes for the self-taught, the stuck, and the silently frustrated offering code clarity without the condescension. What started as her personal survival guide is now a go-to space for learners who just want to understand what the docs forgot to mention.