Dovecot

Create System User
User vmail will be used to store mail related data

If the command complains about the UID or GID being in use, just increment the number. It can be anything between 500 and 999.

Activate
Edit file  and make sure the only included authentication mechanism is passwdfile.

Edit file  and set option

Create file

Add a User
First generate an encrypted password for the user

Now add a line to

Mail Storage
Configuration of the mail storage is done in

Fail2Ban
Create file  with the following content