Mailer-Demon

Somehow a website went out to my whole contact list and I did not send it. It said "mailer-demon", "failure to deliver". I checked for viruses and it was clean. I am getting emails from over 30 people on my contact list asking if I sent it. It was a clean website that was on the front page of my yahoo mail. Any idea why this happened and how to stop it. These people are getting mad. Thanks.

Did you run Malwarebytes?

And have you got some antivirus protection installed on your system - Malwarebytes will help you deal with anything that may have got into your system but you also need something to stop it getting in to start with. If you don't have, I recommend Microsoft Security Essentials - free from here:

Download: Microsoft Security Essentials - Microsoft Download Center - Download Details

Seems to be a recent re-emergence of an old trojan that compromises your email account and sends mail to everyone in your contact list.
My brother-in-law in Connecticut just got hit by this a couple days ago.
He was sent an email by a known person who had already been compromised. He opened the email subject line "Hey" which contained a few words of text something about "You really have to read this" and then a single link to
something that looked like a News article, the URL of the address contained something like whatevernews15.net
I know this because in addition to him having called me, I was in his contact list and received two such emails from him myself.
I decided just for fun to click on the link and found that it took me to a bogus news article about some lady somewhere earning tons of money from home and then was promptly alerted by Malwarebytes Pro that it had blocked access from / to a malicious site (or words to that effect).
So, another vote for Malwarebytes Pro. Well worth the purchase price for a lifetime license. Which I use in tandem with Microsoft Security Essentials as mentioned above.
As an aside, seems every year or two Yahoo mail, gets whacked with some bot or other that performs similarly. Do you or someone you often receive email from "Yahoo"?
Regards
Randy

Yes I did thanks!