Sender Verify Callout in Exim

What is the Sender Verify Callout?

Sender Verify callout is a feature in Exim that helps ensure the authenticity of the sender’s domain by performing a real-time DNS and mail server check. When enabled, Exim attempts to contact the sender’s domain to verify whether it exists and is currently accepting email.

Why Use Sender Verify Callout?

• • Protection against spam: Helps block fake sender addresses.
  • Phishing prevention: Stops forged domains from reaching users.
  • Improved deliverability: Helps prevent rejection by providers like Gmail that check sender validity.
    
    

Understanding the Error: Sender Verify Callout Not Completed

This error indicates that Exim failed to validate the sender’s domain. Possible reasons include:

• • The domain doesn’t exist.
  • The MX record is invalid or missing.
  • The mail server isn’t accepting inbound mail.
  • A firewall is blocking the callout.

Troubleshooting Steps

1. 1. Use WhatsMyDNS.net to check DNS/MX records.
   2. Send a test email from a different domain.
   3. Ask the domain’s administrator to check the mail server’s status.
   4. If all else is fine, investigate Exim's configuration or raise a support ticket.

Command-Line Examples for Exim Admins

Use the following commands to test Exim sender verification:

# Basic address routing test
exim -bt [email protected]

# SMTP handshake debug
exim -bh test.remote.server.ip

# Full routing with debug
exim -d-all -bt [email protected]
  

Add the following options to the Exim configuration to enable verification:

# Exim Configuration Snippet
sender_verify = true
sender_verify_reject = true
  

Drawbacks

• Latency: Each email is delayed by a DNS lookup.
• Firewall Blocks: Ensure outbound port 25 and DNS lookups are allowed.

Recommendations

If you frequently encounter this issue:

• Work with the email sender to fix their DNS and server setup.
• Ensure your firewall allows outbound Exim connections.

Still having issues? Contact our support team to explore secure, paid options that ensure delivery even when sender verification fails.