Troubleshooting Email Delivery Failures in Exim

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Exim is a popular mail transfer agent (MTA) used on many Unix-like systems. When email delivery fails, it is crucial to identify the root cause quickly. This guide covers common causes of email delivery failures and provides step-by-step troubleshooting methods to resolve them.

1. Understanding Common Causes of Email Delivery Failures

a) DNS Misconfigurations

  • Incorrect or missing MX records
  • Improper SPF, DKIM, or DMARC records
  • Failure to resolve recipient domain

b) Blacklisting

  • IP address or domain listed on public RBLs (Real-time Blackhole Lists)
  • Poor email reputation due to spam complaints

c) Incorrect Routing

  • Misconfigured transport rules
  • Incorrectly defined routers
  • Failure to route internal or external emails properly

2. Analyzing Exim Logs

Exim maintains detailed logs that are essential for diagnosing email delivery issues. The primary log files are:

  • /var/log/exim/mainlog: Records all email transactions.
  • /var/log/exim/rejectlog: Logs messages rejected by Exim.
  • /var/log/exim/paniclog: Logs critical errors.

Example Commands to Analyze Logs:

# Check recent email transactions tail -f /var/log/exim/mainlog # Search for errors grep -i 'error' /var/log/exim/mainlog # Find a specific message by ID grep -i 'message-id' /var/log/exim/mainlog

3. Step-by-Step Troubleshooting

a) Verify DNS Configuration

  1. Check MX Records:
dig mx example.com

Ensure the output lists the correct mail servers.

  1. Check SPF Record:
dig txt example.com | grep spf
  1. Check DKIM Signature: Inspect the headers of a sent email to verify the DKIM signature.
  2. Check Domain Resolution:
nslookup example.com

b) Check for Blacklisting

  1. Manual Lookup: Use tools like mxtoolbox.com or:
host 2.0.0.127.bl.spamcop.net
  1. Check IP Reputation: Use sender reputation tools like SenderScore or Microsoft SNDS.

c) Analyze Routing Configuration

  1. Review Exim Configuration:
exim -bP
  1. Test Email Routing:
exim -bt user@example.com
  1. Send a Test Email:
echo 'Test email' | mail -s 'Test' user@example.com

4. Common Errors and Solutions

a) “Unrouteable address”

  • Cause: Misconfigured routers or DNS issues.
  • Solution: Verify router configuration in /etc/exim/exim.conf and ensure domain resolves correctly.

b) “Relay not permitted”

  • Cause: Incorrect relay settings.
  • Solution: Ensure the domain is listed in local_domains or relay_to_domains.

c) “451 Temporary local problem”

  • Cause: Insufficient disk space or resource limits.
  • Solution: Check disk usage and system resources.
df -h free -m

5. Useful Tools for Troubleshooting

  • exim -qff: Force a queue run for frozen messages.
  • exim -bp: Check the message queue.
  • exiqgrep: Filter the message queue.
  • swaks: Send test emails with specific parameters.

Example usage:

swaks --to user@example.com --server mail.example.com

By systematically analyzing logs, verifying DNS records, and using these troubleshooting techniques, you can quickly diagnose and resolve Exim email delivery issues.

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