A problem common among several these days that their Yahoo or Hotmail email keeps sending spam email messages without their consent to all contacts saved in the email.
It means that your email account password have been hacked. Usually hacked of those who are using Microsoft Windows and installing several useless Internet toolbars and other emoticons etc stuff. Not all software should be trusted they track your passwords then send them to software owners.
Once your friends tell you that they are receiving spam emails from your email account and you know you aren't sending them then try these solutions.
You need to change your password, and then the problem should stop because that's mostly what solves this issue.
Run antivirus scan on your computer. I recommend free antivirus software such as AVG and Avast.