If one thanks of a postal letter being mailed, a sender writes the To Address on the envelope so it will get delivered to the correct person. In the upper-left hand corner the writer will write the From/ Return address. The From/Return address does not have to be the same as the sender’s postal address. A sender could place any address there especially if they do not want to be known as the sender. The Return Address would point blame, if a bad letter is sent, to a totally different person. This is an example of spoofing an address. If the letter is not deliverable to the To Address then the post office will send it back to the Return Address. The person with the Return Address will be making the statement, “What? I never sent a letter to that person.”
This concept is the same with sending an email. A sender can send the email to a To Address and spoof the From/Return address to a different person then themselves. If the email is not deliverable, the Return email address will receive the email and its user will be making the statement,” What? I never sent an email to that person.”
So if you are receiving emails notification of emails that you never sent, more than likely someone is using/spoofing your email address From/Return address.
This spoofing is how spammers are able to send large amount of spam and not be caught.