To add a new mail account login to your control panel for the respective domain name.
Open Mail Accounts page and click Create Mail Account button. A Mail Account Properties page like the below will open

Type a mailbox name in the E-mail Address box and select a domain from the drop-down box.
Type password for the new mail account.
Limit the amount of disk space that this mailbox can use. To do this, in Mailbox Size limit box enter the size option
Fill in your personal information.
You can enable an Autoresponder for this email account and set Mail Forwarding from the next available options.
Click Save and your Mailbox is ready.