Mail Operations (Postfix) Cheatsheet

Postfix is the most common Mail Transfer Agent (MTA) on Linux, responsible for routing and delivering email.

Installation

# Debian / Ubuntu
sudo apt install postfix
 
# RHEL / CentOS / Fedora
sudo dnf install postfix
 
# Arch Linux
sudo pacman -S postfix

During installation on Debian/Ubuntu, select "Internet Site" when prompted for the mail server configuration type.

Service Management

sudo systemctl enable --now postfix    # Start and enable on boot
sudo systemctl restart postfix         # Restart after major config changes
sudo systemctl reload postfix          # Reload config without restarting
sudo systemctl status postfix          # Check service status

Configuration (`/etc/postfix/main.cf`)

Key directives in the main configuration file:

# Hostname and domain
myhostname = mail.example.com
mydomain = example.com
myorigin = $mydomain
 
# Network interfaces to listen on
inet_interfaces = all                  # Listen on all interfaces
inet_protocols = ipv4                  # Use IPv4 only (or "all" for dual-stack)
 
# Trusted networks (allowed to relay mail through this server)
mynetworks = 127.0.0.0/8 192.168.1.0/24
 
# Destination domains this server accepts mail for
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
 
# Mailbox location
home_mailbox = Maildir/                # Deliver to ~/Maildir/ (Maildir format)
 
# Relay host (route all outgoing mail through another SMTP server)
# relayhost = [smtp.relay.com]:587

After editing, always reload:

sudo postfix check                     # Check config for errors
sudo systemctl reload postfix

Checking the Mail Queue

mailq                                  # List all emails currently in the queue
postqueue -p                           # Same as mailq (Postfix native)
postqueue -j                           # Show queue in JSON format

Managing the Mail Queue

# Force delivery of all queued emails
postqueue -f
 
# Force delivery for a specific domain
postqueue -s example.com
 
# Delete a specific email by Queue ID (get ID from mailq)
sudo postsuper -d <Queue-ID>
 
# Delete ALL emails from the queue (use with extreme caution)
sudo postsuper -d ALL
 
# Delete all messages in the "deferred" queue only
sudo postsuper -d ALL deferred
 
# Put a message on hold (prevents delivery attempts)
sudo postsuper -h <Queue-ID>
 
# Release a held message
sudo postsuper -H <Queue-ID>

Viewing Mail Details

# View the content (headers and body) of a queued email
sudo postcat -q <Queue-ID>

Troubleshooting (Logs)

Mail logs are the primary tool for diagnosing delivery problems.

# Debian / Ubuntu
tail -f /var/log/mail.log
 
# RHEL / CentOS
tail -f /var/log/maillog
 
# Using journalctl
journalctl -u postfix -f

Common Log Status Terms

status=sent — Successfully delivered.
status=deferred — Temporary failure (e.g., greylisting, network error). Postfix will retry.
status=bounced — Permanent failure (e.g., invalid address, rejected by recipient server).

Useful Postfix Commands

# Show all current Postfix configuration (defaults + overrides)
postconf
 
# Show only non-default configuration values
postconf -n
 
# Show a specific parameter
postconf myhostname
 
# Set a parameter via command line (edits main.cf)
sudo postconf -e "relayhost = [smtp.relay.com]:587"

Cheatsheets