Email hacked/virus

For the past 2 days we have been receiving "undeliverable" and "failure" notices in our Hotmail/Outlook inbox for emails sent entitled "SUP", "Good Morning", "Good Evening" etc.    I immediately changed our password yesterday & have done multiple virus & Malware scans. We have Microsoft Essentials running at all times, and yet people are still receiving these SPAM messages from us. Strangely, there is no record in our "Sent" folder to show that these emails were sent from our email, as with another time we were hacked.  I changed our password again this morning, and still we are receiving notes from friends saying that they are getting these emails. 

What else can we do to stop this????