WP Sending Emails on It's Own

Hi,

So my WordPress site has sent out three emails to all registered users. I did not send the email myself, it went out on it’s own. On two of the three emails, the message was a previous post from months before. One of the three emails contained a post which went up that day.

Does anybody know how this could have happened? And how would I disable any future emails from being sent if I just wanted to do that?

Thanks.