Software

I can't tell you how to change your security information at Yahoo. You'll have to read Yahoo FAQ's or contact Yahoo directly for answers to your questions.

But, I can tell you that spoofing your email address is VERY common and changing your password won't stop it. It happens to all of us. Once someone finds an email address that is valid, they "pretend" to be you while sending out their spam and they don't have to have access to your email account to do it. They just put your email address in their FROM information. Then, they sell lists of valid email addresses to other spammers, so the problem multiplies. The only way to get off those lists is to either delete the email account, or (if you use an ISP) make it inactive for about a month so it drops off the lists.

If you get bounced messages from addresses that you didn't send email to, just throw it away like the rest of us do. If you want to go to the trouble and expense of trying to track down who's using your email address to send email, that's up to you.