vmailmanager is a Postfix / Dovecot / Sogo Account management web panel. It manages a MySQL User Account database.
This guide is one of the best I've found on the internet. https://thomas-leister.de/en/mailserver-debian-stretch/
It's a PHP Yii2 Application. Instructions will be available soon, but basically follow a Yii2 application installation You need Apache or Nginx with PHP installed
put this application in some path for the web server to publish it: /srv/vmailmanager should be a good choice
Be sure to have composer
Change the config/db.php (copy the config/db.php.example and change accordingly)
composer install
./yii migrate
Setup apache or nginx and head to: http://Server IP/vmailmanager/
Have also a look to the "doc" folder which contains configurations examples for Postfix / Dovecot and Sogo