On Debian/Ubuntu systems, Exim4 uses a split configuration system that can be confusing at first glance. The main components are:

/etc/exim4/
├── exim4.conf.template
├── update-exim4.conf.conf
└── conf.d/
    ├── acl/
    ├── main/
    ├── router/
    └── transport/

The uppercase variables (like MAILDIR_HOME_MAILDIR_LOCATION) are macros that can be defined in several places:

1. update-exim4.conf.conf: This is the primary configuration file that gets processed first.
2. Environment variables: When running update-exim4.conf
3. Debconf database: Through dpkg-reconfigure exim4-config

To change these configuration macros, you have several options:

# Method 1: Edit update-exim4.conf.conf directly
sudo nano /etc/exim4/update-exim4.conf.conf
# Add or modify lines like:
MAILDIR_HOME_MAILDIR_LOCATION='/custom/maildir/path'

# Method 2: Use dpkg-reconfigure
sudo dpkg-reconfigure exim4-config

# Method 3: Set them during update
sudo MAILDIR_HOME_MAILDIR_LOCATION='/alternate/path' update-exim4.conf

The numerous if statements serve several important purposes:

• Flexibility: Allows different configurations for different setups (local delivery vs. remote)
• Debian packaging: Makes it easier to maintain different configuration scenarios
• Customization: Lets administrators override defaults without modifying template files

Let's implement a custom Maildir location across the configuration:

# First, edit the configuration file
echo "MAILDIR_HOME_MAILDIR_LOCATION='/mnt/storage/maildirs'" | sudo tee -a /etc/exim4/update-exim4.conf.conf

# Then update the configuration
sudo update-exim4.conf

# Verify the change
sudo grep -r "MAILDIR_HOME_MAILDIR_LOCATION" /etc/exim4

When troubleshooting, these commands are invaluable:

# Check the effective configuration
exim4 -bV

# Test a specific router or transport
exim4 -brt user@example.com

# Verify the configuration syntax
exim4 -C /etc/exim4/exim4.conf.template -bV

• Always run update-exim4.conf after making changes
• Keep customizations in update-exim4.conf.conf rather than modifying template files
• Use version control for your configuration changes
• Test configuration changes with exim4 -bh before applying them

The Exim4 mail server on Debian-based systems uses a unique split configuration structure:

/etc/exim4/
├── exim4.conf.template     # Master template file
├── update-exim4.conf.conf  # Configuration generator settings
└── conf.d/                 # Modular configuration fragments
    ├── main/               # Core configuration
    ├── acl/                # Access control lists
    ├── router/             # Routing configuration
    └── transport/          # Delivery methods

The uppercase variables (like MAILDIR_HOME_MAILDIR_LOCATION) can be defined in multiple places:

# Primary sources:
1. /etc/default/exim4
2. /etc/exim4/update-exim4.conf.conf
3. Directly in /etc/exim4/exim4.conf.template
4. Runtime through debconf system

For persistent changes, edit the update-exim4.conf.conf file:

# Example: Change maildir location
dc_eximconfig_configtype='internet'
dc_other_hostnames='example.com'
dc_local_interfaces='127.0.0.1'
dc_readhost=''
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost=''
MAILDIR_HOME_MAILDIR_LOCATION='/var/mail/%u/Maildir'

After modifying, regenerate the configuration:

sudo update-exim4.conf
sudo service exim4 reload

The extensive use of .ifdef/.endif statements serves several purposes:

• Allows flexible configuration for different deployment scenarios
• Enables easy customization without modifying core files
• Supports the Debian package maintainer's configuration system
• Provides a clean way to handle distribution-specific defaults

Let's modify the maildir delivery location and create a custom transport:

# In /etc/exim4/conf.d/transport/40_exim4-config_maildir_home
maildir_home:
  driver = appendfile
  .ifdef CUSTOM_MAILDIR_PATH
  directory = CUSTOM_MAILDIR_PATH/$local_part
  .else
  directory = /var/mail/$local_part/Maildir
  .endif
  maildir_format
  delivery_date_add
  envelope_to_add
  return_path_add
  group = mail
  mode = 0660

Then define the variable in update-exim4.conf.conf:

CUSTOM_MAILDIR_PATH='/mnt/storage/mail'

To verify your configuration:

# Test configuration syntax
sudo exim4 -bV

# View expanded configuration
sudo exim4 -bP | less

# Test specific router or transport
sudo exim4 -brt user@example.com