In the last two weeks or so on 3 different occasions I have logged in to find several undeliverable notices to emails that I know I haven't sent. On the first two occasions I changed my password and security question; the third occasion was today and I am wondering if it is worth changing my security details if it is just going to happen again.
What are some of the subject lines for the messages which could not be delivered? If you still have some of these - you may have deleted them - then look inside the messages about email not being delivered, mail which you did not send, to see what the subjects were and if you recognize any of the email addresses to which the messages were sent.
Let know one way or the other because as malsiluk has said, there are unfortunately many people suffering similar ongoing problems, and we can help,