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/main.cf /etc/postfix/main.cf.bak
cp /etc/dovecot/dovecot.conf /etc/dovecot/dovecot.conf.bak

Nginx
Create a snippet to use in server block configuration
vim /snippets/ssl-domain.name.conf
with text:
ssl_certificate /etc/letsencrypt/live/domain.name/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/domain.name/privkey.pem;

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

Postfix
use postconf to directly update config file
postconf -e smtpd_tls_cert_file='/etc/letsencrypt/live/domain.name/cert.pem'
postconf -e smtpd_tls_key_file='/etc/letsencrypt/live/domain.name/privkey.pem'
postconf -e smtpd_tls_CAfile='/etc/letsencrypt/live/domain.name/.pem'

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

Restart all services and you're good to test.