Configuring Postfix Relay to Office 365 on Ubuntu

Introduction:

Setting up a reliable email server is crucial for efficient communication in any organization. Postfix is a widely used mail transfer agent (MTA) that can be configured as a relay to route emails to external email providers, such as Microsoft Office 365. This step-by-step guide will walk you through the process of configuring Postfix to relay emails to Office 365 on an Ubuntu server.

Prerequisites:

• An Ubuntu server with root or sudo access.
• A valid Office 365 account.

Step 1: Update Ubuntu:

Begin by updating your Ubuntu server to ensure that you have the latest software packages and security updates.

sudo apt update
sudo apt upgrade

Step 2: Install Postfix:

Install Postfix using the following command:

sudo apt install postfix

During the installation, choose the “Internet Site” option and enter your server’s domain name when prompted.

Step 3: Configure Postfix:

Edit the main Postfix configuration file:

sudo nano /etc/postfix/main.cf

Add or modify the following settings:

relayhost = [smtp.office365.com]:587
smtp_tls_security_level = encrypt
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous

Step 4: Create the SASL Password File:

Create a file to store your Office 365 login credentials:

sudo nano /etc/postfix/sasl_passwd

Add the following line, replacing your_email@example.com and your_password with your Office 365 email address and password:

[smtp.office365.com]:587    your_email@example.com:your_password

Save the file and then hash it to enhance security:

sudo postmap /etc/postfix/sasl_passwd

Step 5: Secure the SASL Password File:

Limit access to the SASL password file to ensure its security:

sudo chmod 600 /etc/postfix/sasl_passwd*

Step 6: Configure the Generic File:

Create a generic file to map email addresses:

sudo nano /etc/postfix/generic

Add the following lines, substituting your_office365_email@example.com with your Office 365 email address:

root@localdomain    your_office365_email@example.com
@localdomain    your_office365_email@example.com
MAILER-DAEMON@localdomain    your_office365_email@example.com

Save the file.

Step 7: Set Correct File Permissions and Hash the File:

Ensure the correct permissions for the generic file and hash it for use in Postfix:

sudo chown root:root /etc/postfix/generic
sudo chmod 0600 /etc/postfix/generic
sudo postmap /etc/postfix/generic

Step 8: Configure TLS:

Enable TLS encryption by editing the TLS configuration file:

sudo nano /etc/postfix/main.cf

Add the following lines to enable TLS:

smtp_tls_security_level = may
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Step 9: Restart Postfix:

After making the necessary changes, restart the Postfix service:

sudo systemctl restart postfix

Step 10: Test the Configuration:

Send a test email to verify that your Postfix relay is functioning correctly:

echo "This is a test email." | mail -s "Test Email" recipient@example.com

Check the recipient’s mailbox to confirm that the email was successfully relayed.

Conclusion: Configuring Postfix to relay emails to Office 365 on an Ubuntu server provides a seamless and reliable email solution for your organization. With these steps, you can ensure efficient email communication while benefiting from the features and security of Microsoft Office 365.