Or stealing $6,000 with $100,000 gun :)
http://www.fenrir.com/free_stuff/columns/callcops/ctc-436.ht...
Regardless, it was some sort of automated spam/phishing emails that were sent from Yahoo's network using my account to contacts on my list. I analyzed the headers of multiple bounced messages that were sent to email addresses no longer in use and confirmed the origin of the traffic.
I'm not going to fall for a phishing attack and I only access email from devices I personally control. Could one of them had some sort of malware infection? I guess it is possible but I am security conscious and it is highly unlikely. I also would expect a hacker that has compromised one of my devices would be far more interested in using my banking credentials than using my Yahoo account to send spam.
The bulk hacking attacks that began around Spring 2010 hit all the big webmail providers. The source of the passwords was always, without fail, reversed hashes from breakins at other big websites:
https://googleblog.blogspot.ch/2013/02/an-update-on-our-war-...
Source: was a tech lead on the Google anti-hijacking team during this period.
Regardless, it's something that has always continued to eat at me since I can't say for certain how it happened.
Most likely either: 1) you were phished and didn't realize it 2) logged in to your Yahoo account from a device that had malware on it