To configure Magento 1.9 to use Postfix as your email service provider, follow these steps:
Connect to your server via SSH or your preferred method.
Install Postfix by running the following command:
sudo apt-get install postfix
During the installation process, you will be prompted to configure Postfix. Choose "Internet Site" as the configuration type, and enter your domain name as the system mail name.
Once Postfix is installed, open the Magento admin panel.
Go to System -> Configuration -> Advanced -> System -> Mail Sending Settings.
Set the "Disable Email Communications" option to "No".
In the "Host" field, enter "localhost" (without the quotes).
In the "Port" field, enter "25" (without the quotes).
In the "Set Return-Path" field, select "Yes".
In the "Return-Path Email" field, enter your email address.
Click "Save Config" to save your changes.
Magento 1.9 should now be configured to use Postfix as your email service provider.