Someone is sending an email to people on your contacts list claiming to be you. They are trying to get your contacts to click a link or send personal information. You are sure that you did not send the message. This is called e-mail spoofing.
What attackers do is to use some email spam software to anonymously send out emails to all the contacts on exploited email accounts. This is sometimes done with an “anonymous remailer” reference: http://en.wikipedia.org/wiki/
Why e-mail Spoof?
The motivation behind it is to conceal the emails original point of creation while at the same time using a list of your contacts (some of whom trust you) to get them to go to a site. That site can have a variety of intentions. Its not always about malware, the site can also be used as an aggressive blackhat marketing campaign to get people to go to a site or sell products and/or service.
But its also used for phishing attacks, network infiltrations, gathering insider information, and getting malware on systems. Whether its advertising or malicious, e-mail spoofing is counting on the user to be unaware of the true nature of the email.
See if you account is just spoofed or hacked AND spoofed. There is a difference. If they are spoofing and somehow got all your contacts.. Honestly, SMTP (email) is not very secure by itself. So there is not much you can do if your email gets spoofed except notify your contacts. Imagine trying to stop someone from sending snail mail to your contacts with a return address that says its from you.. how do you stop that? How would you even find out where they sent it from?
If you account is being actively infiltrated and THEN spoofed there is something you can do.
The first thing you should do is change your password.
A spammer may have altered your account information, allowing them to access your account again even after you change your password.Visit your Account Information page. — yahoo
Click Choose how Yahoo contacts you under “Contact Information.”Change the email address on file if you don’t recognize it.– For help, see adding an alternate email address to your account.Ref: https://help.yahoo.com/kb/