apt-get update
apt-get install software-properties-common
add-apt-repository ppa:certbot/certbot
apt-get update
apt-get install certbot

certbot certonly --standalone -d your-domain.tld

nano /etc/dovecot/conf.d/10-ssl.conf
FILE-ADD:
ssl = required
ssl_cert = </etc/letsencrypt/live/your-domain.tld/fullchain.pem
ssl_key = </etc/letsencrypt/live/your-domain.tld/privkey.pem
FILE-END!

nano /etc/postfix/main.cf
FILE-ADD:
smtpd_tls_cert_file=/etc/letsencrypt/live/<Domain-Name>/fullchain.pem
smtpd_tls_key_file=/etc/letsencrypt/live/<Domain-Name>/privkey.pem
smtpd_tls_loglevel=1
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
FILE-END!