About 10 days ago I received the first of a series of bounce back (undeliverable) emails on one of my secondary comcast.net email addresses. After examining the body of these emails, it appears that my address was used to send something to literally dozens of addresses with the syntax firstname.lastname@example.org, email@example.com or firstname.lastname@example.org ... I immediately ran a full virus/malware scan on my Windows 7 PC (Kaspersky Internet Security), updated my ASUS router's firmware and changed all of my email passwords on the Comcast server (and synced on my iPad). Within several days the bounce back emails stopped arriving, but I'm most concerned about HOW this might have happened in the first place and what further steps you'd suggest I take. This clearly was not spoofing but a compromise of the address. Note that it's highly unlikely this could have been a physical breach or word of mouth. I've been sending/receiving email since the 1980's (seriously) and can't remember a security-related issue that's left me feeling more uneasy than this. Thanks.
Most likely, your email address was 'spoofed' as the sender. You'll need to check the headers of the returned emails to see where they actually were sent from.
I concur with LoPhatPhuud . If you look at the raw image, you will find the
X-Originating-IP. copy the I.P address and paste it in your browser. It will tell you where it comes from. When you find these, mark them as spam.