Email certificates

Use this configuration to apply a Certificate to complete set for mail services

Certificate Files
After obtaining the cert, you will have the following PEM-encoded files:

cert.pem: Your domain's certificate
chain.pem: The Let's Encrypt chain certificate
fullchain.pem: cert.pem and chain.pem combined
privkey.pem: Your certificate's private key

Before you begin do a backup of config files:
cp /etc/nginx/sites-available/web-domain.conf /etc/nginx/sites-available/web-domain.conf.bak
cp /etc/postfix/ /etc/postfix/
cp /etc/dovecot/dovecot.conf /etc/dovecot/dovecot.conf.bak

Create a snippet to use in server block configuration
vim /snippets/
with text:
ssl_certificate /etc/letsencrypt/live/;
ssl_certificate_key /etc/letsencrypt/live/;

add snippets to
server {
listen 443;
include snippets/;
include snippets/ssl-params.conf;

use postconf to directly update config file
postconf -e smtpd_tls_cert_file='/etc/letsencrypt/live/'
postconf -e smtpd_tls_key_file='/etc/letsencrypt/live/'
postconf -e smtpd_tls_CAfile='/etc/letsencrypt/live/'

update dovecot.conf file as following
ssl = required
ssl_cert =

Restart all services and you're good to test.