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Overview

This article provides instructions for configuring Postfix to prevent authenticated users from sending emails with a From: address that does not match their authenticated login credentials. Implementing this restriction helps mitigate email spoofing and enforces sender authenticity within your mail environment.

Note: This procedure is intended for experienced system administrators.

Prerequisites

• Root or sudo access to the server
• Postfix mail transfer agent installed (Plesk default configuration)
• Command-line interface access

Configuration Steps

1. Open the Postfix main configuration file located at /etc/postfix/main.conf using a text editor.
2. Locate the smtpd_sender_restrictions parameter. Modify the value to include reject_authenticated_sender_login_mismatch immediately before the permit_sasl_authenticated directive.
   
   Default Plesk configuration:

   smtpd_sender_restrictions = check_sender_access hash:/var/spool/postfix/plesk/blacklists, permit_sasl_authenticated

   Updated configuration:

   smtpd_sender_restrictions = check_sender_access hash:/var/spool/postfix/plesk/blacklists, reject_authenticated_sender_login_mismatch, permit_sasl_authenticated

3. Save and close the file.
4. Configure the required sender login map by executing the following command:

   postconf -e "smtpd_sender_login_maps = hash:/var/spool/postfix/plesk/virtual"

5. Apply the changes by reloading the Postfix service:

   postfix reload

Verification & Testing

To verify that the restriction is functioning correctly, perform a test transmission using an authenticated user attempting to spoof a different sender address.

1. Create a sample email file:

   echo "this is a test" > message

2. Execute the following command, replacing test@example.com with a valid local mailbox and password with the corresponding user credentials:

   curl -k --url "smtp://127.0.0.1" --mail-from "spoofing@example.com" --mail-rcpt "test@example.com" --upload-file ./message --user 'test@example.com:password' --ssl

3. A successful restriction will return the following error:

   curl: (55) RCPT failed: 553

4. Verify the rejection in the mail logs. Navigate to Warden > Logs > Reject log and confirm the presence of the following entry:

   <spoofing@example.com>: Sender address rejected: not owned by user test@example.com

Troubleshooting

• Restriction not applying: Verify that reject_authenticated_sender_login_mismatch is positioned correctly within the smtpd_sender_restrictions list. Postfix evaluates restrictions sequentially, and placement before permit_sasl_authenticated is critical.
• Map file errors: Ensure the smtpd_sender_login_maps parameter points to a valid hash database file. If necessary, regenerate the map by running postmap /var/spool/postfix/plesk/virtual.
• Service reload failures: Check system mail logs for syntax errors or configuration conflicts during the Postfix reload process.