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Install sendmail service on Debian

Overview

This guide explains how to set up authenticated email sending from the command line on Debian-based systems (including Proxmox Backup Server) using msmtp (a lightweight SMTP client) and s-nail (a mailx-compatible utility). This is ideal for system notifications, backup/email scripts, and environments with DMARC/SPF filtering where authenticated sending is required.

1. Install Required Packages

apt update
apt install -y msmtp s-nail

2. Configure msmtp

  1. Create/Edit the global configuration file:
    nano /etc/msmtprc
  2. Example /etc/msmtprc: 
    
defaults
auth           on
tls            on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile        /var/log/msmtp.log
syslog         LOG_MAIL

account        default
host           mail.hosting.de
port           587
from           admin@simmy.org
user           admin@simmy.org
password
    • Use only plain ASCII spaces (no tabs or Unicode spaces).
    • Replace <your-mail-password> with your actual password.
    • Important: "from" and "user" should match your authenticated email address for DMARC/SPF.
  3. Set strict permissions: 
    chmod 600 /etc/msmtprc

3. Configure s-nail or mailx to use msmtp

Add the following line to your /etc/s-nail.rc (system-wide) or ~/.mailrc (per user):

set mta=/usr/bin/msmtp

4. Send a Test Email

Use the mail command to test sending:

echo "This is the body" | mail -s "Test Subject" recipient@example.com

On success, no output is shown. Check /var/log/msmtp.log or /var/log/mail.log (if syslog is enabled) for debug info if not delivered.

5. Troubleshooting

  • If mail arrives in Junk/Spam, create a filter at your destination mailbox to whitelist the sender or move to Inbox.
  • If you see an error like “account default was already
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