how to prevent Email Identity theft?

I just received an email from my yahoo to gmail account. But I didnt send it from my yahoo account. I checked the mail header and it doesnt say that the email is signed by yahoo. It doesnt say who exactly sent that email and by using which software. I’m afraid my email address is being falsely used without my notice. Is there a way for me to determine who sent that email? how can i stop this?

Please help me.
My yahoo address is in the From field. thats what worries me.

Number 1, do not open the email. If you have, run a scan of your computer. Number 2, you are not in any real danger unless the your yahoo account is in the "from" field. Otherwise, just delete the email and keep browsing the web.

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