If the email is not opened and simply deleted, you will receive a Not Read message. Remember to keep in mind that recipients can choose not to send a read receipt. A copy of the read receipt is received once the recipient opens the email message to view it. The Undo Send feature is a handy way to ensure that the wrong email doesn't get sent to the wrong person. Complete the rest of your email as normal and send the email Read Receipt. If not, you can simply close the message, and it's relegated to your Drafts folder where you can keep it or delete it.
Certain email programs offer an option to recall or retract an email, but that doesn't necessarily mean the recipient won't still receive it. We've all sent emails on the spur of the moment that we've regretted. Should you start looking for a new job? Not if you've enabled Gmail's Undo Send feature. Oops, you've just sent off an angry email to your boss from your Gmail account. You can stop that Gmail email from being sent.