If you wish to delete  an e-mail address associated with your site, like yourname@yourdomain.com, Do this:

Login to Plesk Control Panel, then  click on Mail Accounts in Mail Menu.

 Search the email id which you want to delete. Select a check box corresponding to the mailbox that you wish to remove. And Click  Remove .

 Select Confirm removal and click OK.

Please logout of Plesk Control Panel when done.