Click MAIL on top, scroll down and the options to add/edit accounts appear. See screenshot below for better understanding.
Then click ADD ACCOUNT option and enter the email account address and click save.
You can click the advanced options to enter other values such as quota, aliase email accounts (so that emails to this alias account will also reach the main email account) and forwarder address.
Once saved you can copy the details on the right hand side (marked in red in the screenshot and send it to the email account user). Screenshot below shows the details.
How to set a catch all email account andHow to set a Antispam/Antivirus and DKIM support for a mail domain.
Options for both questions above can be seen at the same page. Click MAIL on top of control panel, scroll down to see the EDIT option, click on it.
Then make required changes and save.
Once done, you can see the saved details displayed in the MAIL area