Your email account was hacked, not your Mac. Email accounts are hacked all the time, usually due to users selecting easy-to-guess passwords. It has nothing to do with what computer you have, as it can happen even if you don't own a computer. Change your email passwords and use long and complex passwords, including special characters, numbers and upper and lower case letters.When I began to receive returned e-mail for over 200 mails I had not sent, I got a little uncomfortable. Just trashing them was not enough. My server closed me down completely and I had to call them and re-do all my passwords on all devices..
I also recommend avoiding Sophos, as it can actually increase a Mac's vulnerability, as described here and here. 3rd party antivirus apps are not needed to keep a Mac malware-free, as long as the user practices safe computing, as described in the following link.
The OP you quoted hasn't been on this site for over a year, so don't hold your breath on getting a response.The thing is how do we know that this is really you posting?