To create an E-Mail account, go to the left side menu and click on "Edit Users". On the next screen, click on "Add a new user to this server".
In the next scree, specify an under the section "Virtual domain user details" an "Email address" (maybe using the surname of the user of the E-Mail address), the "Real name" and a "Password". A password get created automatically. Alternatively, you can type in your own password, if you want. Leave the following settings and sections as it is. Only select under section "Other user permissions" what you want to set up. As "Login permissions" select from the drop down menu "Email only". When done, click "Create".
You should see the new E-Mail user account in your users list.