You should be able to click on your sent items folder in your Email client or Webmail and select the emails you wish to delete.
Or are you talking about deleting an email that you have sent someone so they never receive the email? If this is the case it isn't an option unfortunately
Having deleted emails from Sent folder, they go to a Deleted folder where you can delete them for good. In Post Office and in email client. Like the Windows Recycle bin. Post Office deletes emails in all folders after 45 days.