Access SWPanel and go to the Dashboard of the service where the email account is located, using the search engine located at the top of the page.
Once in the Dashboard of the service, you must go to Service Management → Mail Management from the blue side menu.
Locate the email account and from the [...] menu click on "Delete this email account".
You must confirm the deletion of your email account by clicking on the red "Delete" button
Finally, accept the deletion of your email account, by clicking on the "Yes, delete" button.