First you need to find the Mailboxes subsection in E-Mail section.
In the upper right corner, click New.
Fill all the required fields and click Ok.
Information about mail servers also can be found here. You need to click Outlook in the upper right corner.
Here you can see all the necessary information which you need.
Now you need to download Microsoft Outlook and create an account there. While creating fill in all fields using the data obtained from ISPmanager.
From this step creation of mailbox is completed.