The files in /var/spool/postfix/deferred/ are not plain text emails but rather Postfix's internal queue format. These files contain:

• Message metadata (envelope information)
• The actual email content (usually in RFC 5322 format)
• Queue management data

Method 1: Using postcat command

The proper way to inspect these files is using Postfix's built-in postcat utility:

postcat -q /var/spool/postfix/deferred/[QUEUE_ID]

Example output:

*** ENVELOPE RECORDS deferred/1B2C3D ***
message_size:            1024
...
*** MESSAGE CONTENTS deferred/1B2C3D ***
From: sender@example.com
To: recipient@domain.com
Subject: Test message
...

To process multiple deferred messages programmatically:

#!/bin/bash
QUEUE_DIR="/var/spool/postfix/deferred"
for file in $(find $QUEUE_DIR -type f); do
    echo "Processing $file"
    postcat -q $file > /tmp/$(basename $file).eml
done

Permission problems: Run commands as root or postfix user:

sudo -u postfix postcat -q /var/spool/postfix/deferred/[ID]

Corrupted queue files: Try reconstructing with:

postsuper -r [QUEUE_ID]

For developers needing to parse queue files directly:

import subprocess

def read_deferred_email(queue_id):
    result = subprocess.run(
        ['postcat', '-q', f'/var/spool/postfix/deferred/{queue_id}'],
        capture_output=True,
        text=True
    )
    return result.stdout

print(read_deferred_email('1B2C3D'))

Remember that directly manipulating queue files can cause mail system instability. Always back up the directory before making changes.

---

When Postfix cannot immediately deliver an email, it stores the message in the deferred queue located at /var/spool/postfix/deferred/. These files aren't in standard mail format but rather in Postfix's internal queue format.

Here are three reliable methods to inspect deferred emails:

1. Using postcat command

The safest way to view deferred messages is using Postfix's built-in postcat utility:

# View a single deferred message
sudo postcat -q $(ls /var/spool/postfix/deferred/ | head -1)

# View all deferred messages
for file in /var/spool/postfix/deferred/*; do
    echo "=== Message: $file ==="
    sudo postcat -q "$file"
    echo ""
done

2. Converting to mbox format

To convert deferred emails to readable format:

# Install necessary tools
sudo apt-get install postfix-milter

# Convert and save to mbox
sudo bash -c 'for file in /var/spool/postfix/deferred/*; do
    postcat -q "$file" >> /tmp/deferred_mail.mbox
done'

3. Using Python to parse queue files

For programmatic access, you can use this Python script:

import os
from subprocess import check_output

def read_deferred_mail():
    deferred_dir = '/var/spool/postfix/deferred/'
    for filename in os.listdir(deferred_dir):
        if not filename.startswith('.'):
            full_path = os.path.join(deferred_dir, filename)
            try:
                mail_content = check_output(['postcat', '-q', full_path])
                print(f"=== Message: {filename} ===")
                print(mail_content.decode('utf-8'))
            except Exception as e:
                print(f"Error reading {filename}: {str(e)}")

if __name__ == "__main__":
    read_deferred_mail()

Permission issues: Always use sudo as queue files are owned by postfix user.

Binary appearance: The raw files contain metadata headers - use postcat instead of direct file reading.

Message relocation: Simply copying to maildir won't work due to format differences.

To requeue all deferred messages for delivery attempt:

sudo postsuper -r ALL deferred
sudo postfix flush

For regular monitoring, consider these commands:

# Count deferred messages
sudo find /var/spool/postfix/deferred/ -type f | wc -l

# View deferred mail summary
sudo mailq | grep deferred