Hi Mario,
You don’t need to configure Postfix and Dovecot.
Install at least: NS 
, Account provider: OpenLDAP module, Email module and one of the following Email groupware: WebTop, SOGo, Roundcube webmail.
The rest of modules, depend on how to will use and where the NS server is placed.
You will find some “How to” regarding to setup an Email server in this forum.
One of these: Howto make a simple mail server