adduser user
usermod -aG sudo user

apt install postfix dovecot-core dovecot-imapd

apt install certbot
certbot certonly --standalone -d mail.example.com
postconf -e 'smtpd_tls_cert_file = /etc/letsencrypt/live/<your.domain>/fullchain.pem'
postconf -e 'smtpd_tls_key_file = /etc/letsencrypt/live/<your.domain>/privkey.pem'

uncomment the following lines:
submission inet n       -       n       -       -       smtpd
syslog_name=postfix/submission
mtpd_tls_security_level=encrypt
smtpd_sasl_auth_enable=yes
smtpd_recipient_restrictions=permit_sasl_authenticated,reject
milter_macro_daemon_name=ORIGINATING

add the following 
mydomain = staffieri.co.uk
home_mailbox = Maildir/
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $myhostname
$icated,reject

uncomment 
listen = *, ::

nano  /etc/dovecot/conf.d/10-auth.yconf
update the following line as below:
disable_plaintext_auth = no
auth_mechanisms = plain login

nano  /etc/dovecot/conf.d/10-master.conf
unix_listener /var/spool/postfix/private/auth {
  mode = 0666
  user = postfix
  group = postfix
 }
 
 systemctl restart postfix
  systemctl restart dovecot.service 
  

apt install php php-cli php-gd php-intl php-fpm php-curl php-imagick 
php-mysql php-zip php-xml php-mbstring php-bcmath -y

mysql -u root -p 
create database roundcube_db;
grant all on roundcube_db.* to roundcube_user@localhost identified by  'roundcube_db_password';
quit

cd /var/www/
wget https://github.com/roundcube/roundcubemail/releases/download/1.4.10/roundcubemail-1.4.10-complete.tar.gz
tar -xvf roundcubemail-1.4.10-complete.tar.gz
rm roundcubemail-1.4.10-complete.tar.gz
mv roundcubemail-1.4.10/ roundcubemail
chown www-data:www-data -R /var/www/roundcubemail

apt install nginx

server {
    listen 80;
    server_name webmail.DOMAIN.com;
    return 301 https://$host$request_uri;
root /var/www/html/roundcubemail;
}
 
server {
    listen 443 ssl http2;
    server_name webmail.DOMAIN.com;
    root /var/www/html/roundcubemail;
    index index.php index.htm index.html;
 
   ssl_certificate  /etc/letsencrypt/live/mail.staffieri.co.uk/fullchain.pem;
   ssl_certificate_key  /etc/letsencrypt/live/mail.staffieri.co.uk/privkey.pem;
 
   location / {
        try_files $uri $uri/ /index.php?$args;
    }
 
   location ~ \.php(?:$|/) {
   try_files $uri =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param PATH_INFO $fastcgi_path_info;
        fastcgi_param HTTPS on;
        fastcgi_param modHeadersAvailable true;
        fastcgi_pass unix:/run/php/php7.3-fpm.sock;
        fastcgi_intercept_errors on;
        fastcgi_buffers 4 256k;
        fastcgi_busy_buffers_size 256k;
    }
}