So someone replied to my ad from this website asking about my vehicles. I replied with some general information hoping for a reply. This morning I had over 100 rejected emails in my inbox that came from my domain name!
Do NOT answer any emails from htharrythorne@gmail.com .
Any idea how I can fix this? Someone is now using my domain name to send out spam! 
Thanks 

I talked to Yahoo tech support. They were able to disable some feature that should fix things. We'll see in a few days.
Awesome and immediate customer service, I'm pleasantly shocked!
Ouch, that sucks. Good luck getting this straitened out
I recently got a SPAM message with my own email address as the sender. It was picked off by my SPAMinator because it only allows persons on my address list through to my inbox and my own email address is not on the list. So it's in the "suspect email" file located on Earthlink's server instead of my own computer.
I guess SPAM filters are getting pretty good at picking off SPAM so the perpetrators are ramping up the offensive.
It turns out there is a setting somewhere in Yahoo's business mail that will block all emails from XXX@yourdomain.com. Anything with XXX other than your email address will be blocked. It also prevents spammers from sending spam from your domain name I was told. Hopefully that is true.
SMTP was never designed to be secure. You can change the "from" field to whatever you want. It's not that someone is actually using your email account to send out spam, they're just using your address in the "from" field.
I just got an email about my car for sale from this email as well. When i searched for the email this forum came up. I unfortunately replied and got a response. Still can't tell if it's a real person, but they're saying the saw my car forums.nasioc.com/forums.
Pretty weird stuff let me know if you hear anything.