ssmtp and sendmail Commands

Introduction

Sending emails from the command line is a valuable capability for system administrators and developers, particularly for automated notifications, system alerts, and application-generated messages. Linux provides several tools for this purpose, with ssmtp and sendmail being among the most commonly used.

This guide covers both tools:

1. ssmtp: A simple Mail Transfer Agent (MTA) designed to deliver mail from a local system to a mail hub (SMTP server). It's lightweight and ideal for systems that need to send mail but don't need to receive it.

2. sendmail: A more comprehensive mail transfer agent that has been a standard part of Unix/Linux systems for decades. The term "sendmail" can refer to both the original Sendmail MTA and the command-line interface that many MTAs provide for compatibility.

Both tools allow you to send emails directly from the command line or from scripts, making them essential for system automation and notification tasks.

Understanding Mail Transfer Agents (MTAs)

Before diving into specific commands, it's important to understand what an MTA is:

• An MTA is responsible for transferring email messages from one computer to another using SMTP (Simple Mail Transfer Protocol)
• Full-featured MTAs (like Postfix, Sendmail, or Exim) can both send and receive mail
• Lightweight MTAs (like ssmtp) typically only send mail, relying on external mail servers for actual delivery

ssmtp: The Lightweight Solution

Overview

ssmtp is a simple mail transfer agent designed to deliver mail from a local system to a mail hub (SMTP server). It's not a full mail server but rather a relay that forwards mail to a proper mail server for delivery.

Key features of ssmtp:

• Extremely lightweight and simple to configure
• No mail queues or daemons to run
• Perfect for systems that only need to send occasional emails
• Can use external SMTP servers (like Gmail, Office 365, etc.)

Installation

Debian/Ubuntu-based Systems

sudo apt update
sudo apt install ssmtp

Red Hat/CentOS/Fedora Systems

# For CentOS/RHEL
sudo yum install ssmtp
 
# For Fedora
sudo dnf install ssmtp

Configuration

The main configuration file for ssmtp is /etc/ssmtp/ssmtp.conf. Here's how to configure it to use a Gmail account as a relay:

sudo nano /etc/ssmtp/ssmtp.conf

Add or modify the following lines:

# The mail server (SMTP)
mailhub=smtp.gmail.com:587

# The address where bounces are sent
rewriteDomain=gmail.com

# The full hostname
hostname=your_hostname

# Use SSL/TLS
UseSTARTTLS=YES

# Authentication
AuthUser=your_gmail_address@gmail.com
AuthPass=your_app_password
AuthMethod=LOGIN

# Email address format
FromLineOverride=YES

Note: For Gmail, you'll need to use an "App Password" rather than your regular password if you have 2-factor authentication enabled.

Basic Usage

Sending a Simple Email

To send a simple email with ssmtp:

echo "This is the body of the email" | ssmtp recipient@example.com

This will send an email with no subject. To include a subject and other headers:

echo -e "To: recipient@example.com\nFrom: sender@example.com\nSubject: Test Email\n\nThis is the body of the email." | ssmtp recipient@example.com

Sending an Email from a File

You can also prepare your email in a file and send it:

ssmtp recipient@example.com < email.txt

Where email.txt contains:

To: recipient@example.com
From: sender@example.com
Subject: Test Email

This is the body of the email.

Sending to Multiple Recipients

To send to multiple recipients:

echo -e "To: recipient1@example.com, recipient2@example.com\nFrom: sender@example.com\nSubject: Test Email\n\nThis is the body of the email." | ssmtp recipient1@example.com recipient2@example.com

Advanced Usage

Sending Attachments

ssmtp itself doesn't handle attachments directly. For this, you can use the mutt command in combination with ssmtp:

sudo apt install mutt
echo "Email body" | mutt -s "Subject" -a /path/to/attachment.pdf -- recipient@example.com

Using ssmtp with Scripts

Here's a simple bash script to send an email:

#!/bin/bash
 
TO="recipient@example.com"
FROM="sender@example.com"
SUBJECT="Automated Email"
BODY="This is an automated email sent from a bash script."
 
echo -e "To: $TO\nFrom: $FROM\nSubject: $SUBJECT\n\n$BODY" | ssmtp $TO

sendmail: The Traditional Approach

Overview

The sendmail command is a standard interface for sending mail that's implemented by various MTAs, including the original Sendmail MTA, Postfix, Exim, and others. When people refer to "sendmail," they might mean either:

1. The original Sendmail MTA (a full mail server)
2. The sendmail command-line interface provided by various MTAs for compatibility

In this guide, we focus on the command-line interface, which is available regardless of which MTA is installed.

Installation

Most Linux distributions come with an MTA that provides the sendmail command. If not, you can install one:

Postfix (Recommended Modern MTA)

# Debian/Ubuntu
sudo apt update
sudo apt install postfix
 
# Red Hat/CentOS/Fedora
sudo yum install postfix
# or
sudo dnf install postfix

During installation, choose "Internet Site" and enter your domain name when prompted.

Original Sendmail

# Debian/Ubuntu
sudo apt update
sudo apt install sendmail
 
# Red Hat/CentOS/Fedora
sudo yum install sendmail
# or
sudo dnf install sendmail

Basic Usage

Sending a Simple Email

To send a simple email with sendmail:

echo "This is the body of the email" | sendmail recipient@example.com

To include a subject and other headers:

echo -e "To: recipient@example.com\nFrom: sender@example.com\nSubject: Test Email\n\nThis is the body of the email." | sendmail -t

The -t option tells sendmail to extract recipient addresses from the message headers.

Sending an Email from a File

You can prepare your email in a file and send it:

sendmail -t < email.txt

Where email.txt contains:

To: recipient@example.com
From: sender@example.com
Subject: Test Email

This is the body of the email.

Sending to Multiple Recipients

To send to multiple recipients:

echo -e "To: recipient1@example.com\nCc: recipient2@example.com\nFrom: sender@example.com\nSubject: Test Email\n\nThis is the body of the email." | sendmail -t

Advanced Usage

Sending Attachments

Like ssmtp, sendmail itself doesn't handle attachments directly. You can use mutt or mail commands:

# Using mutt
echo "Email body" | mutt -s "Subject" -a /path/to/attachment.pdf -- recipient@example.com
 
# Using mail (mailutils)
mail -s "Subject" -A /path/to/attachment.pdf recipient@example.com < body.txt

Using sendmail with Scripts

Here's a simple bash script to send an email:

#!/bin/bash
 
TO="recipient@example.com"
FROM="sender@example.com"
SUBJECT="Automated Email"
BODY="This is an automated email sent from a bash script."
 
echo -e "To: $TO\nFrom: $FROM\nSubject: $SUBJECT\n\n$BODY" | sendmail -t

mailx/mail Command

Both ssmtp and sendmail can be used with the mail command (provided by the mailutils or mailx package) for a more user-friendly interface:

# Install mailutils
sudo apt install mailutils
# or
sudo yum install mailx
 
# Send an email
echo "This is the body" | mail -s "Subject Line" recipient@example.com
 
# Send with attachment
mail -s "Subject Line" -a /path/to/attachment.pdf recipient@example.com < body.txt

Common Use Cases

System Notifications

One of the most common uses for command-line email is sending system notifications:

# Send disk usage alert
df -h | mail -s "Disk Usage Report for $(hostname)" admin@example.com
 
# Send when backup completes
tar -czf backup.tar.gz /important/data && echo "Backup completed successfully" | mail -s "Backup Status" admin@example.com

Cron Job Notifications

Cron jobs can be configured to send their output as emails:

# In crontab
MAILTO=admin@example.com
0 2 * * * /path/to/backup_script.sh

Or you can explicitly send email from within the script:

#!/bin/bash
OUTPUT=$(run_some_command)
echo "$OUTPUT" | mail -s "Cron Job Result" admin@example.com

Application Alerts

Applications can send alerts via command-line email:

#!/bin/bash
if [ "$ERROR_DETECTED" = true ]; then
    echo "Error details: $ERROR_MESSAGE" | mail -s "Application Error Alert" admin@example.com
fi

Troubleshooting

Common Issues with ssmtp

Authentication Failed

If you see authentication errors:

• Check your username and password
• For Gmail, ensure you're using an App Password if 2FA is enabled
• Verify that your SMTP server allows relay from your IP address

Connection Refused

If connection is refused:

• Check if the SMTP server is running
• Verify the port number is correct
• Check if a firewall is blocking the connection

Common Issues with sendmail

Mail Queue Issues

If emails are stuck in the queue:

• Check the mail queue: mailq
• Try to force processing: sudo sendmail -q
• Look at mail logs: sudo tail -f /var/log/mail.log

DNS Issues

If you see DNS-related errors:

• Verify your system's DNS configuration
• Check that your hostname resolves correctly
• Ensure reverse DNS is properly configured

Security Considerations

Password Security

• Never store plain-text passwords in scripts
• Use environment variables or secure credential storage
• Consider using dedicated service accounts for sending emails

Email Encryption

• Consider using TLS/SSL for email transmission
• Be aware that email content is generally not encrypted unless you specifically use encryption tools

Preventing Spam

• Limit who can use the mail sending capabilities
• Implement rate limiting for email sending
• Monitor for unusual email sending patterns

Comparison: ssmtp vs. sendmail

Conclusion

Command-line email tools like ssmtp and sendmail provide essential functionality for system administrators and developers who need to send automated emails from Linux systems. While ssmtp offers a lightweight solution perfect for simple forwarding to a smart host, sendmail (or a modern alternative like Postfix) provides a more complete mail handling system.

For most modern systems that just need to send occasional emails, ssmtp is often the preferred choice due to its simplicity. However, for systems that need to handle more complex mail operations, a full MTA with the sendmail command interface is more appropriate.

By mastering these command-line email tools, you can effectively implement automated notifications, system alerts, and application-generated messages in your Linux environment.