Postfix (SMTP Relay)

Setting up Postfix as a SMTP relay

[!NOTE] This setup is based on tranquil IT's tutorial.

Install and enable postfix and it's dependancies :

apt install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules
systemctl enable postfix

Edit the nano /etc/postfix/main.cf file and add/change the following lines :

relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/cacert.pem
smtp_use_tls = yes

Create an application password from your gmail account (2FA needs to be enabled).
Create the /etc/postfix/sasl_passwd file and add your authentication information :
```
[smtp.gmail.com]:587 USERNAME@gmail.com:MDP_Application
```

Fix your file permissions and map the file for postfix :

chmod 400 /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd

Create a certificate for postfix

cd /etc/ssl/certs
openssl req -newkey rsa:2048 -new -nodes -x509 -days 3650 -keyout key-for-smtp-gmail.pem -out cert-for-smtp-gmail.pem
cat /etc/ssl/certs/cert-for-smtp-gmail.pem | sudo tee -a /etc/postfix/cacert.pem

Restart the postfix service
```
/etc/init.d/postfix reload
```

Test and you're done :

echo "Test mail from postfix" | mail -s "Test Postfix" adresse@mail.com

[!IMPORTANT] To make the SMTP server available to your homelab services, you will need to make sure that the /etc/postfix/main.cf file contains the following entries :
mynetworks = 127.0.0.0/8,<your services subnet>
inet_interfaces = all