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It's highly recommended to enable SSL/TLS on your server, both for the webapp and email server.
This doc will use https://letsencrypt.org to get a free SSL certificate for app.mydomain.com that's used by both Postfix and Nginx. Letsencrypt provides Certbot, a tool to obtain and renew SSL certificates.
To install Certbot, please follow instructions on https://certbot.eff.org
As of today (March 25 2020), you can install Certbot by using these commands:
sudo apt-get update
sudo apt-get install software-properties-common
sudo add-apt-repository universe
sudo add-apt-repository ppa:certbot/certbot
sudo apt-get update
sudo apt-get install certbot python-certbot-nginx
Then obtain a certificate for Nginx, use the following command. You'd need to provide an email so Letsencrypt can send you notifications when your domain is about to expire.
sudo certbot --nginx
After this step, you should see some Certbot lines in /etc/nginx/sites-enabled/simplelogin
Now let's use the new certificate for our Postfix.
Replace these lines in /etc/postfix/main.cf
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
by
smtpd_tls_cert_file = /etc/letsencrypt/live/app.mydomain.com/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/app.mydomain.com/privkey.pem
Make sure to replace app.mydomain.com by your domain.