Sending Email from Linux with mailx via Internal SMTP

mailx (heirloom-mailx) and s-nail are lightweight CLI tools for sending email from scripts, cron jobs, and system alerts. They’re especially useful in corporate or campus networks with internal SMTP servers that don’t require authentication.

Installation

Debian/Ubuntu:

apt install heirloom-mailx

RHEL/CentOS/Fedora:

dnf install mailx

Alpine:

apk add mailx

Arch:

pacman -S s-nail

s-nail is a modern POSIX-compliant fork of mailx with better MIME support and is preferred on newer systems, though the command syntax remains compatible with the examples here.

Basic Usage: Single Command

Send email without any configuration file:

echo "Email body content" | mailx -v -s "Subject Line" \
  -S smtp=smtp.example.com \
  -S from="sender@example.com(Sender Name)" \
  recipient@example.com

Replace smtp.example.com with your SMTP server, sender@example.com with your sending address, and recipient@example.com with the destination.

The -v flag enables verbose output for debugging connection issues.

Send to multiple recipients:

echo "Content" | mailx -v -s "Subject" \
  -S smtp=smtp.example.com \
  -S from="sender@example.com(Name)" \
  user1@example.com user2@example.com user3@example.com

Configuration File Setup

For repeated use, add settings to ~/.mailrc:

set smtp=smtp://smtp.example.com
set from="sender@example.com(Your Name)"
set sendcharsets=utf-8

Then send email more simply:

echo "Message body" | mailx -s "Subject" recipient@example.com

Restrict permissions to prevent credential exposure:

chmod 600 ~/.mailrc

Reading Email Body from Files

Send file content as the email body:

mailx -v -s "Backup Report" admin@example.com < /path/to/report.txt

Pipe file contents directly:

cat /var/log/app.log | mailx -v -s "Log Report" logs@example.com

Send output from a command:

df -h | mailx -v -s "Disk Report" admin@example.com

Adding Attachments

Include attachments with the -a flag:

echo "Report attached" | mailx -v -s "Monthly Report" \
  -S smtp=smtp.example.com \
  -S from="sender@example.com(Name)" \
  -a /path/to/report.pdf \
  recipient@example.com

Multiple attachments require multiple -a flags:

echo "Files attached" | mailx -v -s "Backup" \
  -S smtp=smtp.example.com \
  -S from="sender@example.com(Name)" \
  -a /path/to/file1.zip \
  -a /path/to/file2.txt \
  -a /path/to/file3.log \
  recipient@example.com

SMTP Authentication

For servers requiring credentials, add authentication settings to ~/.mailrc:

set smtp=smtp://smtp.example.com
set smtp-use-starttls
set smtp-auth=login
set smtp-auth-user=username@example.com
set smtp-auth-password=your-password
set from="username@example.com(Display Name)"

Or on the command line:

echo "Content" | mailx -v -s "Subject" \
  -S smtp=smtp.example.com \
  -S smtp-use-starttls \
  -S smtp-auth=login \
  -S smtp-auth-user=username@example.com \
  -S smtp-auth-password=your-password \
  -S from="username@example.com(Name)" \
  recipient@example.com

Security note: Storing plaintext passwords in config files is risky. Use environment variables or a credential file with 600 permissions as a minimum, or consider using a local mail relay service like postfix with null authentication instead.

For OAuth2 or other modern auth methods, use -S smtp-auth=xoauth2 with appropriate token setup, though local SMTP relays are simpler for most sysadmin use cases.

Port Selection

• Port 25: Standard SMTP, no encryption (legacy, often blocked outbound)
• Port 587: SMTP with STARTTLS (recommended for authenticated relays)
• Port 465: SMTPS (SSL from connection start, less common but supported)

Specify port with the smtp setting:

-S smtp=smtp://smtp.example.com:587

Enable STARTTLS for port 587:

-S smtp-use-starttls

Troubleshooting

Run with -v to see detailed connection logs:

echo "Test" | mailx -v -s "Test" \
  -S smtp=smtp.example.com \
  recipient@example.com

Connection refused: Verify the SMTP server hostname and port. Check firewall rules allow outbound SMTP. Test connectivity with:

nc -zv smtp.example.com 25
telnet smtp.example.com 587

Authentication failed: Verify credentials and authentication method (login, plain, cram-md5). Check if the SMTP server expects STARTTLS before authentication.

Timeout: Ensure network connectivity. Check if firewalls or ISPs block outbound port 25 (common). Try port 587 or 465 instead.

Message not sending: Review verbose output for SMTP error codes. Verify the from address is acceptable to the SMTP server.

Practical Examples

Daily log digest via cron:

0 2 * * * echo "Daily logs attached" | mailx -v -s "Daily Report $(date +\%Y-\%m-\%d)" \
  -S smtp=smtp.example.com \
  -S from="monitoring@example.com(System)" \
  -a /var/log/app.log \
  admin@example.com

Disk usage alert:

#!/bin/bash
DISK_USAGE=$(df / | awk 'NR==2 {print $5}' | sed 's/%//')
if [ "$DISK_USAGE" -gt 80 ]; then
  echo "Disk usage at $DISK_USAGE% on $(hostname)" | mailx -v \
    -s "ALERT: High Disk Usage" \
    -S smtp=smtp.example.com \
    -S from="alerts@example.com(System)" \
    admin@example.com
fi

Backup completion notification:

#!/bin/bash
BACKUP_FILE="backup-$(date +%Y%m%d).tar.gz"
tar -czf "$BACKUP_FILE" /important/data && {
  echo "Backup completed successfully at $(date)" | mailx -v \
    -s "Backup Complete: $BACKUP_FILE" \
    -S smtp=smtp.example.com \
    -S from="backup@example.com(Backup System)" \
    -a "$BACKUP_FILE" \
    backups@example.com
}

Database backup with compression notification:

#!/bin/bash
DUMP_FILE="db-backup-$(date +%Y%m%d-%H%M%S).sql.gz"
mysqldump -u user -p database | gzip > "$DUMP_FILE"
SIZE=$(du -h "$DUMP_FILE" | awk '{print $1}')
echo "Database backup completed. File size: $SIZE" | mailx -v \
  -s "DB Backup: $DUMP_FILE" \
  -S smtp=smtp.example.com \
  -S from="dba@example.com(Database)" \
  -a "$DUMP_FILE" \
  dba@example.com

Integration with Monitoring Tools

Use mailx in custom monitoring scripts or cron jobs. For Nagios/Icinga or other monitoring systems, wrap it in a notification script:

#!/bin/bash
# notification_wrapper.sh
RECIPIENT="$1"
SUBJECT="$2"
MESSAGE="$3"
SMTP_SERVER="smtp.example.com"

echo "$MESSAGE" | mailx -v -s "$SUBJECT" \
  -S smtp="$SMTP_SERVER" \
  -S from="monitoring@example.com(Alerts)" \
  "$RECIPIENT"

Call from monitoring alerts or as a custom notification handler.