Mystery Emails

I have been recieving emails from the address ‘postmaster@hotmail.com’ informing me that an email i tried to send could not be sent. The problem is that i never sent the email in question.
Also, when i try to contact the email address that it saysi tried to send an email to, i find out that the address does not even exist.
I frequently get these messages. and it is not a security issue, as i have changed my password regularly.