How to correct Postfix' 'Relay Access Denied'?

The "Relay Access Denied" error in Postfix typically occurs when Postfix receives an email from an external sender and determines that it should not relay the email to its destination. This error message is a security feature to prevent unauthorized users or spammers from using your mail server as a relay.

To correct the "Relay Access Denied" error in Postfix, you need to configure your Postfix server to allow legitimate users or hosts to relay email through it while rejecting unauthorized relay attempts. Here are the general steps to do this:

1. Check Postfix Configuration:
   • First, make sure your Postfix configuration is set up correctly. Key configuration files are typically located in the /etc/postfix/ directory on most Linux distributions.
   • Check the main.cf file for settings related to relay control. Look for parameters like smtpd_relay_restrictions and smtpd_recipient_restrictions.
2. Set Up Authentication:
   • To allow authorized users to relay through your server, you can set up authentication mechanisms such as SMTP AUTH (SASL). This requires users to provide valid credentials before relaying mail.
   • Configure SASL by editing the main.cf file and adding or modifying parameters like smtpd_sasl_auth_enable, smtpd_sasl_security_options, and smtpd_sasl_path.

3. Configure Relay Restrictions:

   • Define relay restrictions in the main.cf file using the smtpd_relay_restrictions parameter. You can specify which hosts or IP ranges are allowed to relay through your server.

   • For example, to allow relaying from a specific network (e.g., a local network), you can add something like:

      

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     smtpd_relay_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
     

4. Reload Postfix:

   • After making configuration changes, you need to reload or restart Postfix to apply the new settings. Use the following command:

      

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     sudo systemctl reload postfix
     

5. Testing:

   • Test your configuration by sending an email through your server with valid credentials if you've set up SMTP AUTH. Make sure it is accepted.
   • Test from an external source to ensure that relay attempts are properly denied.

Remember that configuring Postfix relay settings requires a good understanding of your server's security needs and the specific requirements of your email system. Always be cautious when allowing relay access to your server, as improper configuration can lead to security vulnerabilities or spam-related issues.

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