Exim is a popular mail transfer agent (MTA) used on Unix-like operating systems. Syntax errors in Exim typically occur when there are issues in its configuration file (exim.conf) or related files.

Common Causes of Exim Syntax Errors

1. Misconfigured exim.conf File

Incorrect syntax in directives or parameters. Missing or misplaced sections.

2. Invalid ACL (Access Control List) Configuration

ACL rules in Exim are strict, and any misalignment can cause errors.

3. Incorrect Variable Usage

Using undefined or misconfigured variables.

4. Incorrect Formatting

Missing colons, semicolons, or indentation issues in configuration lines.

5. Updates or Incompatibility

Configuration files may break after an Exim version update.

6. Third-Party Modifications

Improper changes made by third-party plugins or scripts.

How to Troubleshoot and Resolve Syntax Errors

Check Exim Configuration

Run Exim built-in syntax checker to identify errors:

exim -bV

Alternative: Check the configuration file explicitly:

exim -C /etc/exim.conf -bV

This will display the configuration details and pinpoint any errors.

Debugging Configuration

Run Exim in debug mode to identify syntax or operational issues:

exim -d -bh 127.0.0.1

This will output detailed information about Exim operations.

Review the Error Logs

Check the Exim error logs for details about syntax or runtime issues:

tail -f /var/log/exim_mainlog

Other log files to check:

  • /var/log/exim_rejectlog
  • /var/log/exim_paniclog

Correct Common Syntax Issues

Missing or Misplaced Colons

Exim directives often require a colon (:) to separate key-value pairs:

acl_check_data:

Ensure every directive has the correct syntax.

Indentation Issues

Some sections, especially ACLs, require proper indentation:

accept:
  condition = ${if eq{$sender_address}{valid@example.com}{yes}{no}}

Check for consistent spaces or tabs.

Misconfigured Variables

Ensure that all variables (e.g., $domain, $sender_address) are defined correctly. For example:

  • If using ${if}, confirm the conditional logic is valid.

ACL Errors

Errors in the ACL section are common. Ensure each block (e.g., accept, deny) is well-formed:

acl_check_rcpt:
  deny:
    condition = ${if eq{$sender_address}{blacklist@example.com}{yes}{no}}
  accept:

Validate Updates

After updating Exim, ensure compatibility:

  1. Verify the Exim version:
    2. exim -bV
  2. Check if the exim.conf file needs adjustments for the new version.

Revert to a Backup

If recent changes caused errors, revert to a working backup of exim.conf:

  1. Check for backups:
    2. ls /etc/exim.conf.bak*
  2. Restore a previous version:
    3. cp /etc/exim.conf.bak /etc/exim.conf

Regenerate the Configuration

If the file is too corrupted, regenerate it:

  1. Use a default configuration file provided by Exim:
    2. cp /usr/share/doc/exim/examples/exim.conf /etc/exim.conf
  2. Customize it to suit your server needs.

Test Changes

After making corrections, restart Exim and test:

sudo systemctl restart exim

Verify functionality by sending test emails and checking for errors.

Preventing Syntax Errors

  1. Validate Configuration Changes:
    • Always use exim -bV to test changes before applying them.
  2. Keep Backups:
    • Backup the exim.conf file before making changes.
  3. Document Modifications:
    • Maintain notes about changes for easier troubleshooting.
  4. Update Carefully:
    • Review the changelog of Exim updates to ensure configuration compatibility.